Extensions can add, modify, and remove functionality from Thunderbird and Firefox. Most users are already familiar with extensions, but you might not know just how many useful extensions are out there. The Firefox Add-ons site has an extensive list of available extensions.
It's worth pointing out that some extensions may not work correctly, work only on some operating systems, or not play well with other extensions. Be sure to read carefully before installing an extension. If you're going to be installing and testing extensions, it's not a bad idea to back up your $HOME/.mozilla or $HOME/.thunderbird directory before you do so, just in case.
Also, be careful about installing extensions from unknown sources. Only install extensions from sites that you trust. Firefox provides some protection by warning users about installing extensions from unknown sites, but you can easily override the warning.
I never used to mind ads on Web sites, until advertisers and webmasters started utilizing intrusive ads that go beyond simple text and banners. You know the ones I'm referring to -- ads that manipulate browser windows, or block page content with overlaid ads that require you to remove them before you're allowed to actually read the page that you've browsed to.
AdBlock extension preferences - click to enlarge
Adblock allows you to block images, scripts, and Flash ads on any page that you visit. It allows you to see all content on a page that may constitute an ad, and set up a filter to block that content, or any content from the originating server. After using Adblock for about a week, I've been able to reduce the number of pop-up ads I have to deal with to almost zero.
Greasemonkey is kind of like a meta-extension for Firefox. Greasemonkey allows scripts to alter Web pages. By itself, Greasemonkey doesn't do much -- but the scripts that Greasemonkey makes possible are exceptionally useful.
The Userscripts.org site has quite a few scripts for Greasemonkey that enable a variety of useful functionality on popular sites. For example, Gmail's interface doesn't provide a delete button -- so the Gmail Delete Button script adds a delete button to Gmail pages.
If you read Slashdot in Threaded mode, you might appreciate the Expand Threaded Comments script, which displays a threaded comment on the page using xmlhttpRequest. Rather than having to navigate forward and back to read comments, you can simply load threaded comments on the main discussion page.
Other Greasemonkey scripts remove curse words from pages, remove ads from popular sites, and provide links to Netflix from IMDB.com movie pages. If you're interested in writing your own Greasemonkey scripts, see the Dive Into Greasemonkey site.
Would you like to boost Firefox's performance? If so, the Fasterfox extension may be just what you need. Fasterfox boosts Firefox performance by tweaking settings like your DNS cache settings, cache, rendering preferences, and the number of pages held in memory for FastBack.
This extension also allows you to enable prefetching -- which can speed up browsing by grabbing content from links that exist on the page you're browsing now. However, be aware this option may not be entirely worksafe, since it may retrieve content from not-for-work sites, even if you don't actually visit those sites.
In addition to its performance tweaks, Fasterfox provides a small timer at the bottom of the Firefox window that displays the amount of time required for a page to load.
One of the most useful extensions I've installed is the del.icio.us extension. It adds buttons to navigate directly to your del.icio.us bookmarks, and to post bookmarks. This makes it easy to use del.icio.us in place of Firefox bookmarks.
The Digg This! extension adds a Digg This! option to the context menu and Tool menu. If you're really a big (bigg?) fan of Digg, you might try the Digg.com Toolbar, which lets you search Digg directly from the toolbar, and includes its own RSS feed reader.
If you want to get real-time feedback on pages you visit, the Blogger Web Comments extension provides a small display in the righthand corner of Firefox. The display shows recent comments about the page, taken from Google's blog search.
Manage your tabs
Tab Sidebar in action - click to enlarge
Firefox 1.5 introduced a few new tab features, but the Tabbrowser Preferences extension adds much more functionality to Firefox's tabs.
One of my favorite features with Tabbrowser is the ability to "lock" a tab. For example, if you do a search on Google, you can "lock" the current tab. Then, when you click on a search result, it will open in a new tab, rather than opening in the current browser tab.
Tabbrowser isn't the only extension to change tab behavior in Firefox. The Tab Mix Plus extension is also worth checking out, as it combines some of the functionality from a number of other tab-modifying extensions.
Tab Mix Plus adds an undo feature for closed tabs, so if you accidentally close a tab for a page you weren't done browsing, it allows you to re-open the tab in the same position, with its page history.
It also allows you to add functions to the Firefox mouse context menu, so you can close tabs, lock tabs, and so forth by right-clicking in any Web page. Tab Mix Plus also has an options dialog that allows you to set each aspect of Tab behavior, from the appearance of tabs to the mouse context menus. In short, it adds a great deal of extensibility to Firefox's tabs.
Finally, if you want OmniWeb-like tabs, there's the Tab Sidebar. Instead of a row of tab buttons at the top of the page, Tab Sidebar provides a Firefox sidebar with thumbnails of each tab.
As a rule, it's a good idea to be careful when mixing extensions that modify the same behavior. However, I've been using the Tab Sidebar and Tab Mix Plus together with no ill effects.
More extensions on page 2 ...
Firefox has some basic feed-reading features, but if you really want to turn Firefox into a full-featured newsreader, give Sage a spin.
Sage handles RSS and Atom feeds, imports and exports OPML feeds, and renders feeds in a "newspaper" type layout that's really convenient. Sage displays all of your subscriptions in a sidebar, and presents the current feed as a Web page. The default style is a bit drab, but the Sage team offers a pretty good set of styles to spruce up your feed-reading. (I prefer the "Plain and Green" style.) If none of those suit your fancy, you can write your own stylesheet.
Sage isn't the only extension that adds feed-reading functionality to Firefox, but it's the best one I've found so far. I installed the Wizz RSS News Reader extension, but it choked when I tried to import my Bloglines subscriptions.
Web developer tools
It shouldn't come as much of a surprise that Firefox developers have created several extensions that are useful for Web development.
Consider the aptly named Web Developer extension. This is the cat's meow for Web developers. It allows you to see and edit a page's CSS, and provides a visual outline for specific page elements, such as tables, block-level elements, and more. It includes validation tools, and allows you to clear cache, HTTP authentication, history, session cookies, and basically provides the kind of fine-grained control over the browsing experience that Web developers need when working on sites. It's also a nice toolbar for users who want extra control over the pages that they visit.
Another extension that is particularly useful for Web developers is the Modify Headers extension. As its name implies, Modify Headers allows you to add, change, and filter HTTP headers. For example, you can use this extension to send a different user-agent string to sites, or to filter out HTTP requests. It can be particularly useful in conjunction with the Web Developer extension, since that extension will display response headers.
The Thunderbird mail client doesn't get quite as much attention as Firefox, but there are a few really good extensions for Thunderbird. Note that if you want to install Thunderbird extensions, you'll need to save them with Firefox and then point Thunderbird to the .xpi file for the extension. As with Firefox, I'd recommend backing up your $HOME/.thunderbird directory before installing extensions. It doesn't happen often, but a conflict between two extensions, or a single bad extension, can prevent Thunderbird from starting or cause it to be unusable.
If you'd like to prevent users from installing extensions in Thunderbird, there's Disallow XPInstall which prevents the installation of extensions. Yes, an extension to prevent extensions -- kind of a Zen concept, if you think about it.
The Quicktext extension comes in handy for anyone who needs to send out form letters or canned responses via email. This extension lets you define templates that you can insert into an email message from a menu, or (even better) using hotkey combinations. Templates can be simple or very complex.
The author, Emil Hesslow, has some documentation on using Quicktext on his homepage. It does take a few minutes to get used to, but it's well worth the effort -- spend an hour or two setting up templates, and you'll save many more hours in the long run.
Enigmail preferences - click to enlarge
Unfortunately, Thunderbird doesn't come with built-in support for GNU Privacy Guard (GnuPG), so it requires a third-party application to enable encryption and signing email. Luckily, there's the Enigmail extension, which provides authentication and encryption based on GnuPG. Be very sure to read the installation instructions for Enigmail before installing it. Enigmail is very picky, and installing the wrong version can cause problems.
You will also need GnuPG set up on your system as well. Enigmail doesn't support Pretty Good Privacy (PGP), so if you're using Thunderbird on Windows, the GnuPG on Windows guide may be helpful.
Installation is the only tricky part -- using Enigmail is easy. Once Enigmail is installed, you can sign messages with your GPG key, encrypt messages, decrypt messages, and verify other people's signatures.
As standalone applications, Firefox and Thunderbird are already top-notch. However, with the judicious application of a few extensions, they can be made to be even more useful.
Of course, this is just a short overview of some of the more interesting Firefox and Thunderbird extensions that you might want to try. If you have experience with a particularly useful extension, add it in the comments.