Firefox 3.5 came out the other day and of course I immediately upgraded. I had been tempted to try the release candidates, but I depend on my browser so much these days that I didn't risk it. Ideally, I'd probably wait until the new version had been out for a while before upgrading.
For the most part, I don't really notice the difference. I don't doubt it's faster, but I haven't really noticed the difference. It seems most people don't notice when something acceptable gets faster, it's when something gets slower that you notice.
One minor annoyance is that when you only have one tab open (and you have the option turned on to still show tabs when there's only one), there is no longer a close button. Probably the thinking was that there is no point in closing the last tab. But I actually used that feature quite a lot when I wanted to keep my browser running, but I wanted to close e.g. Gmail so I wouldn't be distracted.
On the Mac it's not so bad because I can just close the browser window (on OS X this leaves the program running) and then click on the dock to open a fresh window when I need it. But on Windows, if you close the window, you exit the program.
There are workarounds available - obviously other people found this annoying too.
It's good for me to run into this problem from the user perspective. I tend to ignore our customers when they complain about minor "improvements" I've made. I have to try to remember how annoying it can be when you're used to working a certain way and it's taken away from you for no apparent reason.
I shouldn't have been surprised, but I was a little shocked when I ran 3.5 for the first time and it told me about all the add-ons that weren't supported. Luckily, none of them were show-stoppers for me or I would have had to figure out how to go back to the previous version.
It would be nice if the installer could tell you which add-ons were incompatible before it started the install process so you could cancel if necessary. Otherwise it would be a painful process to go through each of your add-ons and try to find out if it runs on the new version.
I guess another option would be to use portable version of Firefox to test add-ons. But even then, you'd be faced with installing them one at a time since there's no way to sync add-ons yet (that I'm aware of). Maybe I need to look at something like Add-on Collections.
One of the add-ons I was surprised wasn't supported on 3.5 yet was Google Gears. Which means I've lost the off-line support in Google mail, calendar, docs, reader, etc. I assume they're working on it.
I've also switched back to Weave to sync Firefox between my multiple computers. I used it for a while before, but switched to Foxmarks because it seemed better. But Foxmarks has turned into Xmarks and doesn't seem to be focusing on synchronization. And Weave has improved a lot. (I originally used Google Browser Sync but that was discontinued.)
One annoyance with these kind of sync tools is that the obvious time to check for changes is when you start the browser. But if you have a master password, then every time you start the browser it asks for your password, which is annoying and also not very good for security.