FF is great for people who like to fiddle with settings and extensions (me) and can be configured to have more features than any other browser.
For most people I think IE is the better choice: all pages work, it's faster, it's more stable, it needs less CPU and RAM and it's more usable than plain, extensionless FF.
^^ I use FF with only the smooth scroll function, it's a fairly minimal browser out of the box which is what I want and IMO still far more productive than IE or any other browser full of functions I'll never use. On my Mac Safari is the obvious choice (I use Camino to open the odd page it can't open) because it's so simple and stripped down yet offers tabbed browsing.
Still sticking with FF but Opera has good bits like the torrents and download manager. Although I can only get this site to open in IE and I thought Opera was supposed to open all websites according to that site linked above?
Well, actually the problem is not in Opera, but in webmasters. For example, putting text onto pictures is wrong, FF tries to correct that text, but opera doesn't.
Not to mention the IE engine isn't only used for websites but also in various places of the Windows explorer shell. It will hardly save you any space anyway.
Just set firefox and thunderbird as your default apps for browsing and mail and you will rarely see them again (OE & IE)
Firefox cannot open some IE specific sites that Mickysoft continues to make IE only , however there is an extension to open any site you want in IE from firefox, That is try to open windoze updates site in FF it wont happen click the open this site in IE option ( extension at work) and it'll always open that site in ie from then on, saves making the retarded sub standard brower (IE) the default I guess