I develop in Firefox mainly, then make sure it's working in IE afterwords. Don't have a Mac around at the moment, but test in Konquerer from time to time (when I can be arsed booting my VMWare up) - and thats pretty similar to Safari.
Just as an aside, put this demo site together for jobhunting - it's sort of a Magelo for NWN.
main site:
http://smkd.net/
sample profile:
http://smkd.net/profile.php?id=24
Still not finished yet, there's an awful lot of data to handle - but in essence it parses save files and lobs the character data into the database. The file parser was interesting to put together - the file format has all sorts of C datatypes that PHP doesn't even know what to do with.
If you check the skill/feat/spell tabs and click on one of the Icons you can see what I was trying to do originally.