How to access Firefox Extension I added in Selenium Webdriver?
It depends on extension. Usually the extension's behaviour can be to some extent controlled by setting appropriate properties (the ones you can find in about:config) when creating an FF profile. For instance to have Firebug window open by default after FF starts I would include the following line in my code:
default_profile["extensions.firebug.allPagesActivation"] = true
The extensions I use usually have some kind of auto-export feature that automatically sends data to server or saves it on disk. I am afraid there is no way of controlling an extension with WebDriver so not all extensions will be usable in automated tests.