Just got the message to upgrade to Firefox 4 Beta 10 and as usual I don't notice any differences - but that's a good thing right?
With 8GB RAM I don't think this will apply so much to me either: "Improvements in memory usage"
My (two) plugins still work too
The ones I use are webmail notifier (gmail, hotmail and a custom squirrelmail script) and adblock plus. Default theme. A cluttered work space is very annoying to me :)