Browsers and sites should ideally support standards. While we developers, designers, and programmers have made great strides (just look at any contemporary site making more than a barebones use of CSS in Netscape 4.7 or earlier and compare with any browser today for example), cross-platform and cross-browser issues still remain due to differences in standards implementation. FF is very new--brand new, if you don't count the beta versions--so it will take time to catch on. Sites shouldn't be developed just to support a specific browser or platform, but until standards are uniformly implemented, we'll probably be stuck with some degree of "looks better in IE 6.0" or whatever. With a 75-80% market share of IE and 90% of Windows PCs at PSW, our bottom line requires this combination as the baseline. But I test on multiple browsers and on both the PC (Windows XP/2000 and even 98) and Mac (OS X, plus others in the company on OS 9). With FF making inroads, I will be looking more closely at FF support. But I think FF may be the most compliant browser on the PC, so support really shouldn't be an issue. The back button issue is one I've seen reported re: multiple sites running different software, so that looks like an FF issue at this point, but it bears looking at.
Any feedback you want to provide--with specifics such as your browser version, OS, Internet connection, page(s) in question, etc.--are welcomed.