Was just celebrating the download day!
Not to mention the bug of forgetting to change the download page content from "Firefox 2" to 3, I've just encountered the first problem with Firefox 3.
I had installed Firefox 3 Beta before, but in a separate folder without replacing Fireofx 2 as my default browser. Firefox 3 Beta considered itself independent and maintained its own bookmarks file "bookmarks.postplaces.html" separate from Firefox 2. But now I've replaced Firefox 2 completely with the new Firefox 3 that when launched it retrieved the empty bookarks file of Firefox 3 Beta that I've never used!
I was astonished for a moment for missing bookmarks - they weren't synced with Foxmarks - but fortunately the main Bookmarks file "bookmarks.html" of Firefox 2 still exists in the profile folder.