openSUSE 10.3 ships with Firefox 2. I switched to Firefox 3 from the "Mozilla" repository a few weeks after it came out (missed the download day/party). So far so good. Most user interface changes are nifty, except for the change of the History menu layout - the history sidebar cannot be enabled from there unlike in previous versions. CTRL-H or the View menu must be used instead. Awkward, but I can live with it thanks to keyboard shortcuts.
Flash-embedding pages (YouTube amongst others) good. No crashes when watching Flash videos although I use a crashy X.org display adapter driver ('radeon'... don't even ask about 'fglrx').
The problem goes when I use seemingly simpler features that I have known since at least Firefox 1.0. I am a laptop user, and I'm often disconnected from the Internet. I use the browser's cache to read pages that I had already skimmed since I can't be bothered to keep zillions of HTML downloads in my home dir. Then problems arise.
Seemingly this version Firefox crashes at random, specially when its session has run for long (t > 30 min.) time and one does stuff in the page view area such as scrolling or clicking on text while the sidebar is active. What a pity, because I like the history sidebar much better than this new separate "Full History" window. More pity is that the cache gets invalidated after a session crash and its contents get wiped out. Of course... I'm not the best person to judge whether this is a bug or a feature, since I don't know much computer security; but I can tell it annoys me to the point I have to be doing backups of the cache et al after closing Firefox successfully:
$ rm -rf ~/.mozilla2 && cp -rf ~/.mozilla ~/.mozilla2
Then if Firefox 3 crashes, I restart it, close it again, and restore the Cache directory from .mozilla2/firefox/SeeminglyHashedSessionId so I can continue reading from it when I'm offline.
By the way, the offline cache (for any browser) seems to be often underestimated. Some time ago, after the www.wesnoth.org server crash, I got some forum pages from Firefox's cache and uploaded them to this website in a hidden directory to serve as a partial, temporary mirror for people for having a guide to get back to work after the 2-months roll back of the forum's database.
The cache issues apart, the fact that Firefox gets crashy for nothing disturbs me. Wasn't this version supposed to be more stable than 2.0 according to the announcements? The rendering also got some performance regressions. Some pages take longer to be *rendered* than downloaded, specially those with heavy use of scripts. Those affected pages usually are also sluggish to scroll up/down, no matter if I disable "smooth" scrolling. I never experienced anything of this with the same pages on the same OS (openSUSE 10.3), the same architecture (x86_64) and earlier version (2.0.0.x) of Firefox.
Perhaps this whole download-day thing was just a trap. Or they put more attention to the Windows and MacOS X ports rather than the GNU/Linux one. Or I am cursed to have bad luck for the rest of my life. Whatever it is, I don't like it, and I'm seriously considering switching to a better open source browser for Linux; IceWeasel may be it if it is a fork that is being developed on its own. I have yet to do the switch to Debian.