If you need to put parental controls on your Firefox (which I do need) then suricate is a really good add on. Basically its the entire suricate user community that creates the database of blocked websites, but it is really easy to make your own white list.
I know most people on here aren't parents and in fact would not want their parents to see my suggestion, but if you are a parent then this is worth a look.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix