Solution is here:
http://superuser.com/questions/332782/middle-click-tab-opening-stopped-working-in-firefox/332794#332794
for me: I simply uninstalled GreaseMonkey
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It was not a firefox bug, but a greasemonkey one
** Changed in: firefox (Ubuntu)
Status: New = Invalid
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https://bugs.launchpad.net/bugs/843680
Title:
middlemouse.openNewWindow
