I'm using ff4b6 right now.

Firefox's page loading speed (especially with ff4b) isn't actually noticeably slower on most sites than Chrome. Until Mr Flash and other plugins come around. That's probably what makes FF feel so horrible right now. Flash video just kills FF performance (just having the video paused in another tab) like nothing else. They seriously need to fix the flash problem. Having flash crash and bringing down my whole browser sucks.

Aside from that, downthemall's problem is the plug in itself. There's definitely something wrong with how the plug in is written, not FF itself. And I believe FF4 tears tabs correctly now.