This hack will allow you to identify your browser as a Googlebot ‘spider’.
Why would you want to do that? Many newspaper and magazine sites that would normally hassle you for a user name and password will let you slide on in if they think you are one of Google’s spiders out gathering data.
OTOH, it can make some web pages look weird or not work the way they should. But you can always change the user agent back to Mozilla or whatever suits you.
Install the Firefox user agent switcher
http://chrispederick.com/work/firefox/useragentswitcher/
In Firefox, Tools->User Agent Switcher->Options, add Googlebot as a user agent:
Description: Googlebot
“Googlebot/2.1 (+http://www.google.com/bot.html)” all without the double-quotes
The remaining 5 lines can be left blank.
Then switch to this user agent with Tools/User Agent Switcher.
Alternative is to go to http://www.bugmenot.com and look up a password, but the user agent hack is a lot quicker and cleaner.
The prefbar (http://prefbar.mozdev.org) can do the same, and has quite a set of other nice features…
Toggle animated gifs on/off, switch off colors or images in the pages (pages from slow sites load tremendously much faster without images), toggle java and java script…
and so on – all important stuff quickly accessible.
Multizilla does the same but is IMO too bloated.