The Vista forums are full of complaints about problems with Firefox. The Firefox guys are working on them. MS is releasing compatibility patches as fast as they can. The last patch fixed around 50 different applications. They still have a long way to. Drivers are on of the bigger problems.
I will never be able to print on our 2 year old Canon copier/printer, as an example. Nor on the HP Color printer either. I have to copy my print jobs to another computer running Windows 2000, then go to that computer and print them out. The only way that is going tochange will happen when we buy new printers to replace the ones we have.
The performance problems with Vista are due to the same hardware running XP Pro will run games about 25 to 35% faster than running under Vista on the same hardware.
If you were getting worse performance under XP, than you get from Vista on the same computer, then you had a very bad installation of XP Pro. It's not an opinion. It is fact. Just as an example. XP Pro had around 230 threads running at any given time. Vista has around 450 (after cleaning it up) threads running.
You cannot run more software without sacrificing performance. It is a physical impossibility.