On Windows, some Firefox installations are global to all users, while others are user-specific.
firefox_windows.go is only checking for the user-specific ones.

The workaround I used was copying the HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla\Mozilla Firefox key to HKEY_CURRENT_USER\SOFTWARE\Mozilla\Mozilla Firefox. Without it, browsh fails with "Error reading Windows registry: The system cannot find the file specified", even if Firefox is actually installed.