You can change user agent in firefox by going to

about:config
Then add a new “String” by right clicking.

Give “general.useragent.override” as name of the string.

For the value, use what you need.
I put

Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)

So when you visit web site, access log will have it like

59.98.40.114 s1.video.bizhat.com – [22/Aug/2008:21:41:00 +0000] “GET /up.html HTTP/1.1” 200 3 “-” “Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)”

Some site give access to Google bot to protected area of the site. So with this User Agent, you may able to find pages that are not available for normal users.