I have recently become a Web Developer (more PHP CMS stuff than CSS HMTL ****e) but what i have noticed a few things. Personally for all decent heavily tested sites and noob sites, OPERA is far the BEST web browser, alot of Firefoxes features have lineage in opera. Opera is poor for CSS testing and Javascript fullstop.
The best browser to develop in is Firefox with Firebug plugin, though IE is more forgiving for a n00b. in terms of Speed, it goes Safari (though nothing works) Opera, IE7, Firefox. Javascript in IE7 is far more Robust than in anything else, alot works where in the others it doesn't.
Operating Systems:
Windows Vista > *. I don't care what anyone says, it is by far the best operating system though u need a faster comp.... i was using it on a P D 3.0ghz, 1gig and couldn't do much, on sat i upgraded to a Core Duo T9300 and 4 gig and it so good!..... the robustness where programs should really crash (as in not responding) and it recovers them. I used Linux and can't really stand it. There is some really good open source software out there but i tend to use the private gear, Dreamweaver over Aptana, CuteFTP over FileZilla, PSCS3 over everything.