I ended up removing all four addons from my system (since they seem to be unmaintained and/or deprecated); here are the package names:
xul-ext-ubufox - Ubuntu modifications for Firefox xul-ext-unity - Firefox extension: Unity Integration xul-ext-webaccounts - Ubuntu Online Accounts extension for firefox xul-ext-websites-integration - Firefox extension: Website Integration -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to unity-firefox-extension in Ubuntu. https://bugs.launchpad.net/bugs/1069793 Title: Can't install search engines (Tramples over window.external) Status in unity-firefox-extension package in Ubuntu: Confirmed Bug description: The addon "Unity Desktop Integration" replaces windows.external with its own object, breaking the ability to install Opensearch plugins from JS ( window.external.AddSearchProvider() ) TESTCASE: 1) Go to http://mycroftproject.com/dlstats.html 2) Choose any of the OpenSearch plugins and click on the name EXPECTED BEHAVIOUR: A modal window should pop up asking if the chosen plugin really should be installed ACTUAL BEHAVIOUR: A modal window pops up stating that the browser does not support OpenSearch plugins To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/unity-firefox-extension/+bug/1069793/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp