Review for User Agent Switcher by efreak1
Rated 3 out of 5 stars
This addon changes the user agent globally (for every site). If you want to just change the user agent on a site-by-site basis, install the UA Control addon: https://addons.mozilla.org/en-US/firefox/addon/uacontrol/
To create your own collections, you must have a Mozilla Add-ons account.