in the url bar type
about:config then right click and click new>>string
in the first box put:
general.useragent.overridethen press ok in the second box put
Googlebot/2.1 (+http://www.googlebot.com/bot.html)then press ok
now when you browse site's you browse as google bot...
To remove google bot just delete the string
Edit:You can get more user agents from here:
[You must be registered and logged in to see this link.]just write whatevers in the first collom in the second box should work