Although I did indeed notice definite speed improvements in FF4 with my day to day browsing and work, I still would love it to be faster. I mean, it's never a bad thing. And it doesn't seem to utilize graphics cards very well like IE9 and Chrome seem to do. So get to cracking Mozilla!
I love firefox but it still has a long way to go. It is very slow with some jquery scripts, actually ff3.6 ran my easing animations better than ff4. And I wish they would do something about the crappy font rendering. Webfonts in Chrome look amazing compared to firefox.
FireFox has gotten better FF4/Aurora(5?) is the first build I consider to be day-to-day useable (With a lot of tweaking).
Over all Opera still wins for speed 90% of the time and best tools, but has too many strange bugs when doing development so you have to switch between FireFox a lot well doing development.
I still admire Opera for the fact that when one somebody elses computer I can download it and 2 Extensions and have a lot of advanced configuration done in a couple of minutes, FireFox is 1-2hrs of finding about 24~30 Extensions (Depending on what you want it to do) and then remembering were all the different option windows are.
my #1 complainant to firefox vs opera for development is FireFox's Cache system.... Cache viewer is an improvement but vs Operas built-in Cache search FF is a epic nightmare.