UPDATE: deleting extensions.json did not work by itself – i ended up rebuilding my Firefox profile and importing on the necessities
In a previous post i offered a quick tutorial that i scarfed from ghacks.net which would re-enable all the add-ons that Mozilla disabled because of their certificate blunder. The file we manipulated in order to achieve this was extensions.json. Well, i started noticing some weirdness in that extensions that i installed post-fix weren't showing up in about:addons after a browser restart.
If you also employed the extensions.json hack to get your add-ons re-enabled, i suggest backing up your profile, deleting extensions.json and restarting the browser. There should be nothing more to do.