Explanation: FF21 has changed the folder where it stores (some of) its extensions.
But Avast was still trying to install AdBlocker in the folder that had been used by FF20 (and earlier), so FF21 didn’t “see” it.

For a full explanation, see http://www.ghacks.net/2013/05/15/why-you-may-have-lost-access-to-plugins-or-extensions-in-firefox-21/