You needn't be concerned with browsers whereas you should be concerned with rendering engines. SeaMonkey 1.1 and Firefox 2 are both based on Gecko 1.8 and thus will render exactly the same. AOL inserts a single line of code and therefor implements the Window's deep-rooted Internet Explorer so if the system IE is IE 5.5 AOL == IE 5.5.
In general there are only a handful of rendering engines to be concerned about...
Gecko (Firefox, Camino, SeaMonkey)
KHTML / Linux
Opera (rendering engine is constantly renamed)
Trident (IE and anything that incorporates IE)
Webkit is a fork from KHTML and I've read what I hold to be a rumor that they may be merged back together.
There are other rendering engines though these five represent the largest shares of browsers on the market. The next largest human/browser group would most likely be more miscellaneous versions (such as an Amiga-OS specific browser) and cellphone/PDA portable browsers (Opera Mini would most likely be in the lead though I don't track portable browser stats for a lack of interest for designing for screens smaller then the textarea I'm currently typing in).
That being said your secondary concern should be the versions of those rendering engines.
Installing IE8 decimated my ability to test with IE 4, 5.0, 5.5, and 6.0. My IE3 (I have just for kicks since it does not support CSS in any meaningful way) and IE7 standalones still works though IE8 allows emulation (though I use IE8 and IE7 standalone side by side anyway as per my preference).
Opera users are typically tech savvy and update their browsers often.
Gecko browser users at least on Windows (and possibly OSX) are usually pretty good about it though Mozilla has been severely slacking on *nix support. I still can't install a nightly build of anything on a *nix OS and if I have to mess with a command line to install a GUI application then forget it!
Any way I recommend for business site testing...
IE6+ (IE 5.0 and 5.5 bugs are the exact same for IE6 so serve an internet explorer conditional comments style sheet (IECCSS) to correct the issues for IE6 and older as so...
Fix IE 5.0, 5.5, and 6.0 with a secondary CSS...
If you don't prefer to run multiple Firefox profiles I'd recommend testing with either Mozilla Suite (the last version uses Gecko 1.7), Mozilla SeaMonkey for Gecko 1.8, Firefox 3 nightly build for Gecko 1.9. Very few people use Gecko 1.7x or older though it suffers from some nasty application/xhtml+xml bugs.
Opera has supported running multiple versions for a while now. Opera 8.02 and newer support AJAX so in essence if your business site uses AJAX it's safe to require 8.02 or newer only as Opera users don't slack on updates.
Hope this gives you better insight in to testing browsers.