When I first tried FF3B1 I'd posted that it's memory consumption went through the roof. After less than 24 hours and an apparent fix on the server side, and using the exact same binary, I can now start up and use FF3B1 without the same issue that caused it to crash yesterday.
My experiences so far while limited have proven to be pretty positive. I've had it up and running over the night (well, since past midnight local Orlando time), and it didn't crash or run up memory. I can't accurately determine if memory usage is less, since right before I stopped it and re-started it it had a memory footprint of 140 MB. Right now, with 11 tabs open, it's at a little over 90MB. I do know that every time I visit Google Mail that the first time that page is opened it immediately jumps a good 20 MB.
Page rendering is the most notable feature of FF3B1. Pages just seem to snap into existence, and the simpler, the snappier. I don't see pages half-render while the busy icon swirls. The pages are briefly busy, then they're there in front of you.
I run with Adblock Plus and Filterset.G Updater. Adblock plus still runs (thankfully) while Filterset.G is disabled (having stopped, oddly enough, at 3.0a9). On the theme side, the only theme I had outside of the default was nuoveXT 2, and it's disabled. I guess if there's a complaint about plugins and themes is that when a change takes place with Firefox (and it's been a long running problem) that renders a plugin or theme incompatible, that it takes a long time to never to get it updated to the latest Firefox release. It makes me wonder why someone in the Mozilla group hasn't produced a process and a tool for migrating plugins and themes from one architectural change to the next. If the original owner wouldn't do it I certainly would.
Of course, being a first beta, this is a bug shake down, and the first bug I ran into is using Blogger's editor. Inside the Blogger editor it appears I can't use the Link tool to add links to text. I tried it twice, then switched back to Firefox 2 to finish this article. There may be other little bugs out there, but this problem with blogging tips me immediately back to Firefox 2. Maybe Beta 2 will be better.