It is dangerous to be sincere unless you are also stupid - George Bernard Shaw
FIREFOX VERSION 3.0.2 has been released to address four security flaws, two of which are labelled Critical, as well as a number of other bugs and several minor enhancements.
Mozzarella has also updated Firefox 2.0 simultaneously to resolve the same set of security vulnerabilities, along with some others, in version 2.0.0.17.
Critical security advisories for Firefox are described as: "Vulnerability can be used to run attacker code and install software, requiring no user interaction beyond normal browsing."
Firefox critical advisory MFSA 2008-41 is described as "Privilege escalation via XPCnativeWrapper pollution," while the second critical advisory MFSA 2008-42 is described as "Crashes with evidence of memory corruption."
Although Mozilla doesn't say that any of the Firefox security flaws fixed with these releases have been exploited yet, all Firefox users should probably upgrade as soon as is convenient.
Mozilla's automatic updating facility in Firefox kicked in when we logged on this morning.
It if didn't for you, the L'Inq provided below should take you to the Firefox 3.0 operating system, country and language-specific website that's appropriate for your particular wibbling circumstances. ยต
See Also
Install
Firefox 3 the right way (for Linux users)
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L'Inq
Firefox
Hi

Just to inform that some upgrades will fail and stored password are not working after upgrade. Fix is to downgrade back to 3.0.1

OskariO
That FF2 update is quite the disaster, I tried it on a FF2 and now the browser crashes left and right, crashes after ten minutes showing a static page, crashes when you select search for updates, crashes when you submit a comment on a site, and when you close it it also 5 out of 6 times instead of closing causes a crash and the windows message 'firefox.exe had to be closed'.
Makes you wonder if they are trying to get rid of FF2 users, although after that they might move to opera rather than FF3.

The FF3 update seems smooth though for me, although I'm missing the 'removed awfulbar' or 'added option to not have the awfulbar' fix :/
Attempted to upgrade to FF3 on home XP system (university student version). Did nothing but crash every time I attempted to start. Finally got to work but couldn't save any bookmarks, etc..

Full uninstall of FF3 and reinstall of FF2 and all is well, except that all my bookmarks are gone.
Apparently, Mozilla doesn't think it's too much of a big deal since they already set the bug to "resolved":

https://bugzilla.mozilla.org/show_bug.cgi?id=454993

In their world, telling someone to downgrade and/or "just live without your passwords for a while" is sufficiant.

http://support.mozilla.com/en-US/kb/Cannot+use+or+save+passwords+after+upgrading+Firefox
I have found 3.0.2 seems to be slower on loading/playing of videos....
oh crap i've already installed it pending a restart.... oh well here goes
3.0.3 is out. It resolves the pw issue and such.