Sometimes the web site just refuses to work with Firefox or Safari because the web developers just don't feel like having to support those browsers, in which case Amiga2000HD's suggestion might work, but more often than not, the site just isn't compatible with the Mac because it uses Microsoft ActiveX or .Net frameworks for scripting, which is only compatible with Windows. In that case, even IE for Mac won't help since IE for Mac doesn't support ActiveX or .Net either.
There are very few websites out there that I run into that don't support Safari, and I haven't run into one for a long time, but if you are unlucky enough to be in the situation where you HAVE to use such a site, running IE for Windows may be your only solution. If possible, try to use an alternate site, say from a competitor, and be sure to send the website's support people a kind email letting them know you have switched to their competitor because of their poor support for internet standards. If they get enough people complaining, they may just change their web development strategy to include non-Microsoft browsers and OSes.
Mac User since 1989
MacBook Pro 15.4"/2.33GHz Core 2 Duo/4GB/250GB HD/256MB VRAM
Mac mini/2.0GHz Core 2 Duo/1GB/120GB HD
PowerMac G4 "Sawtooth"/1.4Ghz G4/1GB/2 x 120GB HD/64MB ATI Radeon 8500
iPhone 3GS 32GB on Rogers Canada
Master of the Art Of Geek.