I would forego the uninstall/install firefox just for now and check what is contained in the about:config first.

I don’t feel this xpinstall.signatures.required option is directly related to XP, though it is quite possible. Just create a new tab to open about:config and in the search window just type either install or signatures and see what it displays.

If it does relate to the OS you installed firefox on, then it may be win7install or something like that.