(I do not work for Mozilla so this is speculation):
As far as I can tell, side loaded extensions require a full, non-automated, review http://i.imgur.com/r070Grv.png and it wouldn't surprise me if side-loaded extensions were the worst offenders.
Upon discovering a malicious extension, Mozilla could look through all extensions they've signed and blacklist (https://addons.mozilla.org/en-US/firefox/blocked/) all extensions with a similar signature similar to how some anti-malware databases work.
There is probably not all that much stopping you from writing a malicious extension that passes the AMO automated review (example: https://addons.mozilla.org/en-US/firefox/blocked/i1058), but the cost for malware writers is going to be significantly higher since it will be far easier for Mozilla to shut them down via their blacklists.
As far as I can tell, side loaded extensions require a full, non-automated, review http://i.imgur.com/r070Grv.png and it wouldn't surprise me if side-loaded extensions were the worst offenders.
Upon discovering a malicious extension, Mozilla could look through all extensions they've signed and blacklist (https://addons.mozilla.org/en-US/firefox/blocked/) all extensions with a similar signature similar to how some anti-malware databases work.
There is probably not all that much stopping you from writing a malicious extension that passes the AMO automated review (example: https://addons.mozilla.org/en-US/firefox/blocked/i1058), but the cost for malware writers is going to be significantly higher since it will be far easier for Mozilla to shut them down via their blacklists.