Browser was working perfectly then in the middle of browsing session it would not react to mousewheel clicks that had been able to open tabs previously. Does not seem to be a problem with the mouse because I can still use it to do the same thing in IExplorer and chrome, in FF it goes to the fast scroll mode. Tried reinstalling but did not want to uninstall and reinstall fearing loss of addons... About:config says browser.tabs.opentabfor.middleclick true middlemouse.contentLoadURL;false middlemouse.openNewWindow;true middlemouse.paste;false middlemouse.scrollbarPosition;false