User Agent Platform Spoofer by defkev

Effectively what general.useragent.override does but automated so you no longer need to update the preference on each Firefox update.

Mainly to mess with Malware sites distributing binaries tailored to your platform based on the User Agent, e.g. *.exe on Windows and *.bin/*.sh on *nix

Spoofing can be disabled selectively by adding the domain (FQDN or TLD) to the Whitelist under Add-ons -> User Agent Platform Spoofer -> Options

There is no button or anything, simply install and forget.