Rated 5 out of 5 stars

Really useful for testing. Unfortunately, I need to some environment sniffing, and the fact that this tool can set both useragent and navigator.platform is great.

navigator.userAgent support is currently broken due to a bug in Firefox, but it looks like that will get fixed in Firefox 22. In the meantime, you can manually go to about:config, set general.useragent.override to what you want, then restart Firefox.