Are you running Greasemonkey? There's a conflict between Greasemonkey 0.9.10 and Tab Mix Plus on Firefox 6.0.2. Until the add-ons site approves Greasemonkey 0.9.11, you'll have to install it manually. [https://addons.mozilla.org/en-US/firefox/addon/greasemonkey/versions/0.9.11 Greasemonkey 0.9.11] [http://turbulentsky.com/middle-clickcmdclick-on-a-link-doesnt-open-tabs-in-firefox-6-0-2.html Middle-click/Cmd+click on a Link Doesn’t Open Tabs in Firefox 6.0.2]