Bookmark Code: about:config?prefname=general.useragent.override&prefvalue=Mozilla%2F5.0+%28N8x0+like+iPhone%3B+U%3B+Linux+like+Mac+OS+X%3B+ARM%3B+en%29+AppleWebKit%2F420%2B+%28Gecko%2C+like+KHTML%29 to set your UA string to it, and Code: about:cofig?prefname=general.useragent.override&prefvalue= to set it back to default.
about:config?prefname=general.useragent.override&prefvalue=Mozilla%2F5.0+%28N8x0+like+iPhone%3B+U%3B+Linux+like+Mac+OS+X%3B+ARM%3B+en%29+AppleWebKit%2F420%2B+%28Gecko%2C+like+KHTML%29
about:cofig?prefname=general.useragent.override&prefvalue=
about:config?prefname=general.useragent.override&prefvalue=