According to the MozillaZine Knowledge Base article Profile folder - Firefox:
Mac
Profile folders are in one of these locations:
~/Library/Mozilla/Firefox/Profiles/<profile folder>
~/Library/Application Support/Firefox/Profiles/<profile folder>
The tilde character (~) refers to the current user's Home folder, so
~/Library is the /Macintosh HD/Users//Library folder.
This section of that article describes the default cache location. You can also see where it is with this url: about:cache
and change it by entering this one: about:config
and then changing the preference named browser.cache.disk.parent_directory.
Update:
I haven't updated the screenshot, but this still works in FireFox 15.0.1.
Here's a screenshot of Firefox 13.0.1 running on OSX 0.6.8 (Snow Leopard). As you can see the cache folder has been put in a custom location on another volume using about:config
.

.../Library/Caches/Firefox/Profiles/hm7m0wcn.default/Cache
.