In case you haven't heard of it, Greasemonkey is a plugin for Firefox that lets you quickly install and run scripts that change the content of selected Web sites. The scripts download and install in seconds, and make it easy to tweak your most visited sites.
Gmail conversation preview
This script, available from persistent.info, enhances Gmail's built-in preview function. By right-clicking on the message in the inbox, you can preview the entire message in a pop-up bubble, with the option to archive it, leave it unread, delete it, or just close the bubble. With the hundreds of emails I receive each day, this script saves me time by allowing me to quickly scan through the content of my new messages without having to wait for new pages to load. It also helps me find specific information I need without running a full search.
Gmail persistent search
If I do need to run a search of my email archives, as I do frequently, this script, also available at persistent.info, comes in handy. It places a toolbox on the left side of the Gmail page that lists links to my recent searches. Often when I am working on a story or looking for information I received some time ago, I run the same search over and over again. I also frequently search for email messages with attachments. This tool makes those searches quicker and easier, because I can save and re-run any search query I like. These "persistent searches" are listed in the same toolbox that shows my recent searches.
One drawback of using a Web mail program like Gmail is that clickable mailto: links still open up Thunderbird or whatever your computer thinks is its default email program. The Gmailto: script, written by Andrin von Rechenberg, fixes that problem by ordering the browser to open a pre-addressed Gmail composition screen instead of the email program on your hard drive. That lets me can click those links instead of copying and pasting. If you'd still like the choice to open Thunderbird or Gmail, another script will give you the option.
This script takes your Google bookmarks and makes a toolbox for them on your Gmail inbox page, over on the left. You'll need to give your bookmarks the label "GMqlinks" for the script to work properly. This one is nice because it keeps my bookmarks with me no matter what computer I am using, and they're right there on Gmail; I don't have to click over to my bookmarks page. I can collapse and expand the bookmarks box just like any other toolbox on Gmail.
These Greasemonkey scripts are so easy to use that you might be tempted to add dozens of them. Be careful: I've found that running more than three or four at a time seems to slow Firefox down.