highlight and right click Firefox

[Maxheadroom]

in firefox if you highlight something and right click you get the option to search google for it or search ebay for it,

is this customizable can you add your own? thanks

 

[Bongo]

I only get what's being used by the search box at the time. Using the drop-down menu for the search box I can change to any of them and that's what I get when I highlight and then right click.
Jerry