I’ve been using the Firefox 3 betas for a few months, and now actual release candidates are coming out of Mozilla HQ. I made the switch because Firefox 2 was causing the load on my work PC to spike randomly , leaving the machine useless for a stretches lasting seconds. The performance tweaks for memory usage and the rest seem to have done the trick, and even the v3 betas were better than their predecessor.
On the user side of things, I really like a lot of the interface improvements that are part of the new version. The AwesomeBar is actually awesome, although it takes a bit to get used to how it works. It replaces the boring old Location bar from every other browser and is smart enough to autocomplete matches based not just the url you are typing in but the title as well. It also seems that more frequently used URLs float to the top of the list of matches. The bookmarking system is more useful, simply clicking on a star in the awesome bar saves a bookmark. I still haven’t used the new tags system to help organize my bookmarks, what I really want is a plugin that replaces the local bookmarking system with del.icio.us, or at least mirrors itself to that link sharing service. There is a lot to like in Firefox 3, if you haven’t upgraded already, you should.
Memory wise, a much needed memory cycle collector is now in place to take care of freeing memory no longer used by modules that requested it but failed to release it properly. This and other memory oriented tweaks, seem to have paid off so far: a set of tests I recently ran suggests a noticeable increase in memory management efficiency with more memory freed as tabs and windows are closed and no mysterious memory eaten up when Firefox is kept open and idle for several hours as it has been reported several times in the past for Firefox 2.