Tor Browser can't bootstrap when launching it with firefox.exe

If we open firefox.exe from its directory instead of using the link the bootstrap fails.

The reason is that TorProcess can't find some files.

I haven't investigated, but probably we're resolving some relative links against the current working directory instead of resolving them in the proper way (i.e., relatively to firefox.exe itself, or in case of data relatively to the profile).

We should fix this, and maybe also fix tor-browser-build#40892 since we're at it.