The Firefox add-on manager has been much improved and revamped in Firefox 4.0. The new page shows featured Personas (now baked in by default), and recommended add-ons and "up and coming" add-ons. It also simplifies managing add-ons and should be a bit more friendly to new users.
Firefox 4.0 also incorporates the Test Pilot feature as a default, rather than a separate add-on. This means that every Mozilla Firefox user is a potential "test pilot," for the project. Don't worry — this is fully opt-in, not opt-out. Users don't have to participate if they don't want, and nothing stealthy is going on. The Firefox folks provide clear notices about the program and the tests that are running. This may not sound like an exciting feature, until you consider how many millions of users are running Firefox that the project can look to for information about Firefox usage. Applied correctly, we ought to see a lot of improvements in future Firefox releases driven by data from Test Pilot studies.
Security and Privacy Improvements
Mozilla has made a few security and privacy improvements in Firefox that most users are unlikely to notice. Despite being stealthy, the features are actually quite important.
For instance, Firefox 4.0 implements HTTP Strict-Transport-Security. What's that? Basically, it forces connections over HTTPS rather than an insecure connection over HTTP. Sites that want to ensure that users are only connecting over HTTPS can rewrite requests that include "http://" to "https://" and redirect users to the secure connection.
Firefox 4.0 also includes support for Do Not Track (DNT). There's good news and bad news about this one. The good news is that Mozilla is part of the effort to pioneer a HTTP header to tell servers not to tell sites to track users. The bad news is that it's not widely implemented, so the DNT header is not doing much for you right now.
User Interface Improvements
A lot of work has gone into improving Firefox's user interface in 4.0. This ranges from things like tab groups to changing the way that the location bar works.
Tab grouping is a biggie. With Firefox 4.0, tabs in a Firefox window can be grouped so that you have the ability to switch between two or more groups of tabs. For example, let's say that you have Twitter, Facebook, and your work Webmail open. It's time to buckle down and get to work, so you hit the tab group button and create one group that has Twitter and Facebook, and another that has your Webmail and maybe another Web app you use for work. You can switch the social media group out of the way while you're answering work mail and doing work, then swap to the Twitter and Facebook tabs (while work tabs go out of sight) on break.
Firefox 4.0 also provides a "pin as app tab" option. App tabs show only the "favicon" for a site (so Gmail would only show the little envelope icon, for instance).
Another UI improvement is the combination reload and stop button. It takes up less space in the location bar, and there's little reason to have two separate buttons — at least according to the Firefox folks. This one took some getting used to, but it's all in the name of reducing unnecessary browser "chrome" and providing more space for the Web.
And there's evidence that the Firefox folks will roll back UI changes that too many users find uncomfortable. Since as long as I can remember, Firefox would display the URL for a link in the bottom status bar. Then during the 4.0 testing cycle, it was moved to the address bar — so hovering the mouse over a link would display the link in the address bar rather than the status bar. Two problems with that — one, it was weird. Maybe this would work fine for users who'd been unused to the previous behavior, but longtime users didn't seem to like it. Two, when displayed in the address bar it usually didn't seem like as much information was displayed.
The final outcome? The status bar is gone, baby, gone. But when you hover the cursor over a link, it displays the URL in the bottom of the browser in a little bubble where the status bar would be. Win on two counts — it clears out space when it's unneeded, and displays the information where the user expects it when it is needed.
There is one unfortunate note to the Firefox 4.0 release for Linux users: We're not getting the same features that our Mac and Windows brethren are getting. Specifically, hardware accelerated graphics and the one-button menu. Because of driver problems on Linux, the Firefox folks have focused on Windows and Mac for accelerated graphics. The one-button menu is apparently available for Linux, but it's not on by default.
Should I Upgrade?
The big question, should you take the time to upgrade to Firefox 4.0? If you're like me, you've already been running the betas for months, so the 4.0 release will just be a minor change. But if you're still using Firefox 3.6 as supplied by your distribution, is it worth jumping through any additional hoops to get the 4.0?
In a word, yes. Ubuntu, openSUSE, Fedora, and other major distros should have packages available through official or unofficial channels close to the Firefox 4.0 release. There's also the tarball offered by Mozilla as well, but unfortunately they don't provide an AMD64 build at this time. Even though you might need to jump through a few additional hoops to add the Firefox 4.0 packages, the new features are well worth it.
Besides, if you wait too long, you'll be updating to Firefox 5.0. At least according to the roadmap, the Firefox folks are planning to do more rapid releases and ship Firefox 5, 6, and 7 in 2011.