type "about:config" in the address bar. search for preference name"browser.tabs.opentabfor.middleclick" set boolean value to "true". if you don't have this, right click->new->boolean - copy the above preference (minus quotes), set value to true. and you should be all set.
you might want to try Tab mix plus. does a lot more than most of the other tab extensions and has been very reliable for me.
there are quite a few bug reports on this
https://bugzilla.mozilla.org/show_bug.cgi?id=313718
https://bugzilla.mozilla.org/show_bug.cgi?id=425285
https://bugzilla.mozilla.org/show_bug.cgi?id=228840
...
just don't seem to gather any interests tho, weird....
Before it got fixed, I would suggest this way.
1. enable bookmark toolbar
2. arrange all your bookmarks into bookmark toolbar folder through bookmark manager
3. middle click or drag bookmarks from bookmark toolbar to the empty space of tabbar to open in new tab in background.