According to the MozillaZine Knowledge Base article Profile folder - Firefox:
Profile folders are in one of these locations:
~/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.
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