As for FF portable, you, 4wd, spoke about that version from the beginning; I have to admit I didn't "get" this in time and installed the normal version, but of course, a cache of 50 MB or suche doesn't explain my loss of space.
A default full installation of Firefox sets the disc based cache at 1024MB, (the same as IE seems to be whenever I've done a new install), this is about 950MB too much AFAIAC.
FirefoxPortable has the disc cache set at 0MB, (but not fully disabled), by default.
What I believe Shades is talking about is the Profile folder which contains more permanent data, eg. bookmarks, passwords, addon data, etc.
The FirefoxPortable link I put in above. (on my server), differs from the official one in the points I mention previously - the folder shouldn't get any larger unless you install more addons.
BTW, the homepage I added when it's run is instructions on using BYTubeD with DTA!, (with pictures).