If you want to block adult contents in your broswer, ProCon Latte Content Filter is a free mozilla Add-on that will help you to block adult contents.It will block websites popping up or being accessed by the user. When it block site it will show a message under Firefox address bar.
You can add websites into whitelist and blacklist whitelisted website can access through browser but blacklisted websites cannot access. You can also set password to lock modifications
Download ProCon Latte Content Filter click here.