Hello UtMansMan, as you already say "''certain servers and sites not being accessible because of adult content''" and not only for that(adult content) but it is [http://www.wikidot.com/faq:private-sites Private Sites], private sites need to log in as a member to have access, as a premium member to have access in specific pages (a simple member does not have the access) etc. another example is "scientific sites", you have access in a small content of knowledge (Abstract only in form of pdf or word) but you have to be a member or to pay to download all the article. so, you can not do that with any browser(not only firefox) thank you