Safari will present problems with htmlarea, pre-moodle 2.0 editor - you will at least get text editor window, but not html editor. However, if you replace the htmlarea code with xinharepla code I believe you can get safari working. The threads have gotten very tangled, but you can search on Mauno's name and html editor.
Firefox will work with htmlarea on any platform I believe as long as the version is 2.x +, so as Marcus noted it will work with firefox on linux, unix, etc.
As far as Moodle working for linux or Mac, are you talking
server side or client side? Client side Moodle does not have its own client, but relies on a browser (though you may need various plugins for various content). Server side should run on any linux and runs on Mac as well, and there are packages for installation on OS X. Server side requirements are covered in the installation doc,
http://docs.moodle.org/en/Installing_Moodle, though there has been discussion that this could be beefed up a bit.....