Add "Open new window in background" feature

[codygman]

I'm not sure if this is possible after searching the web for "open new window in background firefox -tab", but it would be nice to have.

I have 3 different cases:

• opening links in new windows and switching to them (covered)
• opening links in new windows in the background
• opening a link on the current page, reusing the current tab

The last one requires a bit of messing with about:config and I'll include it just in case any solution for opening a new window in the background conflicts. Basically you must set:

browser.link.open_newwindow.restriction = 0
browser.link.open_newwindow = 1