The
UserAgent Switcher addon is the one I use. It default to the original setting, but only once per session. I.e., if you open the browser, set the UA and then don't quit the UA will be set to that whenever you quit. This particular addon comes with a basic set of UAs, but the support website has a link to a list of further UAs you can download.
Further, as I mentioned it's easy to set your own because it has a custom setting. What I do is visit whatsmyuseragent.com in the browser I wish to copy a UA from. That site gives me the UA, I select it, copy it, go back into the Mozilla based browser and paste this in to the setting in UserAgent Switcher.
Sometimes you want something else though, so doing a Google search for useragents of the type you are looking for (say Firefox useragents) can give you a list of Macs/PCs running various versions of FF. Copy a string and paste into the setting in the addon.
Note. User Agents extensions seem to be a cottage industry for Mozilla addons. So if the addon I linked to is not your preference, there are many, many others. There's at least one out there I've seen that allows you to set a specific UA based on the websites you visit.