Something for you to play with:
(Right-click->Save as...) (NLA)
Just extract and run the FirefoxPortable.exe, all I've done is:
- made sure it's always in Private Browsing mode, (all history/cache/cookies/cake/etc is cleared on exit),
- made sure disc cache is disabled, (it uses 50MB of RAM as cache),
- added BYTubeD and DTA! and configured them.
You might get a couple of tabs open asking if you want to install the Java Quick Starter and Microsoft .NET Framework Assistant - your choice as to what you do but No is preferable.
So, in theory, it'll only ever use about 65MB of disc space for the executable folder plus whatever you download into wherever you download it.
To uninstall: Delete the folder.
It also has a nice homepage.
BTW, concerning your "lost space" problem, check for the following folders:C:\Documents and Settings\<user>\Application Data\Mozilla
C:\Documents and Settings\<user>\Local Settings\Application Data\Mozilla
If you don't have Firefox, (and possibly SeaMonkey), installed then you may safely delete those two folders.
FirefoxPortable will create them again when run, (FirefoxPortable only puts a few small files in them unlike a full Firefox installation), and delete them when it exits.