This great extension (v1.5.9) doesn't change Javascript useragent ! And window size (resolution) spoofing doesn't working too ! Breaks youtube !

AddOn is a good idea - but the Dev got like 3 Addons, some of them overlaying - hope this get´s cleaned up in all his publishings. Problem: Running this breaks JavaScript entirely, even if you want to run JS, you need to disable to addon (Symbol in FF is no toggle for this, like with other AddOns). I will see in the future again if some update cleans up they issues :)