Review for User Agent Switcher by Paul J.
Rated 4 out of 5 stars
Probably not the normal case usage though. But this lets you get around dumb filtering decisions. One star off as you have to hunt around for agent profiles that aren't included, but those aren't terribly hard to find.
However this combined with FFMpeg, lets you watch HTML5 HD via WebM while still eking out life on a machine with WinXP as the OS. (Don't ask how I figured that out.) Apparently some content or content quality is blocked if those sites don't think you'll meet their ideal requirements, and this add-on allows for a workaround.
To create your own collections, you must have a Mozilla Add-ons account.