Firefox has traditionally been a pig of a program, and so it doesn't surprise me to see them drop entire chunks of code in preference to a rewritten component. This is the smart thing to do.
I backed up my
~/.mozilla directory and I downloaded 3.5 beta4.
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1b4) Gecko/20090423 Firefox/3.5b4
I simultaneously installed the Tree Style Tab add-on (add-on, home page). It gives me a left sidebar. I have a wide screen, so I have lots of width to work with. I don't particularly like the big grey column I get, since I don't have many tabs loaded. I'm also still getting used to my eye going to the left instead of the top to see where I currently am. Still, I'm enjoying it quite a lot.
So far I don't have much to say about this new version of Firefox. It doesn't seem much faster to me, but it does seem to be avoiding its 100% CPU usage with Flash. My power supply fan doesn't go nuts anymore, but I see the usage does frequently spike just under 100% when there's flash being interacted with. I guess I'll take what I can get on this one.
I still have to disable version checking to get some essential add-ons to work, like ReloadEvery. It ticks me off that I can't do this on an individual add-on basis. Oh well.