Check for user install of Firefox on Windows

[whimboo]

As reported in mozilla/geckodriver#1045 we currently only check for the system installation of Firefox on Windows. This is done via the registry by using keys under HKEY_LOCAL_MACHINE.

If the user installs Firefox only for him/herself the registry entries will instead be located under HKEY_CURRENT_USER. But this key is never obeyed.

Maybe we should first check for the user install, and then the system wide install?

[andreastt]

Some more details in mozilla/geckodriver#1045 (comment).