some websites and forums will block unregistered users, but they won’t block Google Bot.

and its possible by firefox,,First grab the User Agent Switcher add-on for Firefox called ‘user agent’ here and install it.
Now go to Tools > User Agent Switcher > Options and then again to Options.




Select User Agent from the left sidebar and click Add. Now in the description field type:
crawl-66-249-66-1.googlebot.com
and in user agent field type:
Googlebot/2.1 (+http://www.googlebot.com/bot.html)
as shown in the screenshot below.



Select Google Bot as your User Script by going to Tools > User Agent Switcher.


Now browse any website or forum without registering.