Firefox doesn't auto-update extensions when a new version of the extension comes out. Only Chrome (probably Chromium too) does that. So, if a Adware/malware company buys Ad-block (as a fictional example) then the adware/malware company can update the extension with a new version that contains adware/malware and Chrome will automatically upload it and install it to the users machine (They already have to have ad-block installed). It is a way to get software installed inside a users machine bypassing all the normal security safeguards.
Again, now that Google knows that this can happen they effectively have to remove the capability before any harm occurs else they can be considered liable.