From MozillaZine Knowledge Base

Finding the profile folder

Using about:cache

Type about:cache to the location bar and hit enter. There you see the location of the disk cache device directory. The profile folder is the parent directory.

I removed the above because it only works (on Windows) when the profile is in a custom location. For profiles in the default location, you get the system temp directory. For example, on Windows XP I see this for about:cache:

C:\Documents and Settings\Alice\Local Settings\Application Data\Mozilla\Firefox\Profiles\tejxcfb6.test\Cache

My profile is here:

C:\Documents and Settings\Alice\Application Data\Mozilla\Firefox\Profiles\tejxcfb6.test

Alice 19:21, 9 November 2009 (UTC)

P.S. I don't have a problem with someone adding back the section as long as they explain the above. Alice 19:31, 9 November 2009 (UTC)

Hey Alice, how are you? Good thing you tried out the advice, looks like the results are different on windoze and GNU/Linux. It seemed like such a straight forward way but no wonder it wasn't already listed... --FatJohn 04:48, 12 November 2009 (UTC)

Hi, John, I'm doing fine, thanks. I didn't realize that Linux shows the actual profile folder path in about:cache, even for profiles in the default location. I checked browser.cache.disk.parent directory (which is linked from the Profile folder - Firefox#Folders section) for related bug reports and found the open Linux bug on that, Bug 239254 - [Linux] Support disk cache on a local path. Mac OS X also uses a different directory for the Firefox cache (bug 216204) under ~/Library/Caches/Firefox (ref.). This page and also this forum post say that the Mac OS X cache is located here:

~/Library/Caches/Firefox/Profiles/*.default/  

Anyway, I'd just as well forget mentioning that you can find the profile folder with about:cache. Firefox 3.6 will be out soon and then it will be simple to find and open the Firefox profile folder, as long as you can open Firefox. Alice 12:08, 13 November 2009 (UTC)

P.S. I edited the Profile folder - Firefox#Folders section to add that Linux includes the Cache folder in all Firefox versions. Alice 12:27, 13 November 2009 (UTC)