User Agent: Mozilla/5.0 (Ubuntu; X11; Linux x86_64; rv:8.0) Gecko/20100101 Firefox/8.0 Build ID: 20111115183541 Steps to reproduce: I recently updated my Firefox install to 8. I'm on Ubuntu and using the version in the package repository. I believe the previous version was 4. Actual results: The middlemouse.contentLoadURL option was reset to false (the default). Expected results: All user-set about:config options should have retained their values. It's not clear whether this is a problem with Firefox or with Ubuntu's packaging; hopefully someone can look into it and contact them if necessary.
Can you find out what was the default in Firefox 4? I suppose you had it on 'true' in that Firefox 4. Have you specifically set it to that?
Is this Bug 660447? Can you see if the workaround there works for you?
Indeed it is Bug 660447. I thought the setting changed only on update, but it was reset on every browser start. Disabling the add-on fixed the problem. Thanks for your help!