Recently I noticed that a number of installed Firefox addons were using a language different from what they used to. The language I had been expecting was some kind of English, but what was being used was some non-English language. Investigating a bit, came across: how can I force addons to be displayed in English?
This sounded similar to the situation here and there didn't seem to be a solution mentioned that didn't involve a fair bit of reinstalling and reconfiguring.
Tried removing addons one by one and eventually discovered that disabling (or removing) a version of the Pencil addon
seemed to fix things.
So perhaps selective addon-disabling/removal is a potentially helpful approach...
For the record, had manually installed the seemingly problematic addon via an .xpi, so perhaps I had that coming as (at the moment anyway) there doesn't appear to be a version compatible with the version of FF here:
Not available for Firefox 18.0