Today I faced similar situation (v137): Firefox wants to update, so I restarted, but didn't allow installer to run on admin rights. FF started as usual, but with prompt i need to restart, so I pressed restart... and over and over - update didn't asked for admin privilleges anymore. Next I just closed Firefox and started it again and it worked now - update asked again for admin rights and installed succesfully. Maybe it's a bug or feature?
