I'm trying out Firefox 3 on my laptop running Ubuntu 7.10. It's a huge improvement over Firefox 2 because of three things: it's very fast, it integrates much better with the rest of the desktop in terms of UI, and it has full-page zoom.
Firefox 3 uses Cairo/GTK2 for rendering all its page elements on Linux. (On Windows and Mac it uses the appropriate toolkits as well.) This means that it looks like any other Gnome application on my computer, and though that may seem like a small thing, it actually benefits the experience tremendously.
Full-page zoom, a feature I've always wanted in Firefox, is finally implemented in the browser. (It was implemented in various extensions for Firefox 2 but they did a pretty bad job.) Full-page zoom is where instead of the just the text size increasing, the images and the layout scale as well. It's worth noting that IE7 and Opera already have this feature.
To install it on Ubuntu Gutsy, I installed the package firefox-3.0. Unfortunately, there is one important flaw in how the Ubuntu people built the Firefox 3 package: it's not built against a version of Cairo that supports detection of the system subpixel font rendering settings. This means that the fonts have ugly color bands around their edges. In theory I could build my own copy of Firefox 3 from source with the --with-system-cairo flag, which would fix the problem, but after a few hours of reading bug reports and forums I found out that Firefox 3 requires at least libcairo version 1.5.2. The Ubuntu Gutsy repositories only include version before that. Turns out the only repository that has the correct version of Cairo is the Ubuntu Hardy repository, which means that I have to upgrade to Hardy, which is still in development, to get the correct Cairo library to be able to build Firefox 3.
I briefly considered upgrading to Hardy but decided it wasn't worth it. But Firefox 3 is good enough that I'm not giving it up just because of that font rendering problem. I'll survive with the poor rendering until Hardy officially comes out in April.
Update 1 minute after I finished writing this: Screw caution. I'm upgrading now.
Update 2008-02-28: The upgrade failed because of package dependencies (something to do with update-manager-core) and I tried to roll back but that messed up a lot of the install. So I'll just make a new partition and install Hardy directly. It's only at alpha 5, but I can manage a little instability. Also, I'm going to use the 64-bit version, so for the first time I'll get to use a 64-bit OS on my computer. Interesting how what started as an innocent attempt to try a new browser ended up in me upgrading my OS entirely... :)