I looked in Firefox's Tools>Add-Ons menu and followed link to Language Help but I couldn't see one that does it on an OnMouseHover but I did see some one-click versions.

Problem is, they're all linked in to Google so you may end up sharing all your personal data with the whole world! :D

Take the route I went down and see what you think.


 
I don't think there would be one for mouse hover as the program would eat up all your CPU cycles every time you moved the mouse around the screen. use the one click solution instead.