Firefox 6 deletes bookmark tags. It took me some time to notice that I had been losing Firefox bookmarks tags. I reported the bug on August 26, 2011. (bugzilla.mozilla.org/show_bug.cgi?id=682512) The status of my report in Firefox Bugzilla (bug tracking site) is marked as verified and fixed (but the fix was not present in Firefox 7 beta) therefore this bug is still present in the version of Firefox 6.0.2
It is the same bug that existed in Firefox 3.6 Mozilla (bugzilla.mozilla.org/show_bug.cgi?id=415389) According to the policy of Firefox's developers, the bug will not be fixed until the release of the of Firefox 7 . That means that Firefox bugs are considered "fixed" when they are not. This allows plenty of time for people "enjoy" Firefox bugs and for hackers to exploit published Firefox bugs. Before you download Mozilla Firefox, ask yourself: "can I trust Open Source software"?
Update: In 2011 update scheme changed, now new releases come out as soon as major bugs fixed, (thanks to me?) support.mozilla.org/en-US/questions/896705