There are many user-agent switcher add-ons for Firefox. Most of them change it globally. There is one called UAControl that allows to specify it on a per-site basis (and does it well).

I wanted one that would be based on Jetpack, the new framework for Firefox add-ons. At least I would learn something. So I made ua-site-switch, which not as complete as its nice competitor yet. The code it there on github (git and github are so nice to use!).