I've just run into this problem. After some reading, and noticing that I had dom-inspector installed (cf bug #65916), I removed 'extensions.cache' from my profile and restarted. After this, I could install/upgrade/etc. add-ons again.
I've attached my extensions.cache file from before and after in a single file. ** Attachment added: "extensions.cache pre and post-regeneration" http://librarian.launchpad.net/6463809/extensions.cache -- can't disable/install/uninstall firefox add-ons https://launchpad.net/bugs/79540 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
