Monitor Master adds functionality for multiple monitors in Firefox. Open, merge and position windows, tabs and links through a graphical control item.
Here is a little more about the add-on from the developers:
Monitor Master enables you to position and merge browser windows, to move and merge tabs, and to open links in other windows and on other monitors. Monitor Master adds a graphical control item to the context menu’s for tabs, links and toolbars and to the menu of its own toolbar button. The graphical control depicts your monitors plus all open Firefox windows.
Need more information? Check out this great add-on at the Firefox Add-ons website.