Which, for 99.99999% of actual users will mean they are blocked. Digging around in the guts to enable "Developer mode", with all the scary consequences, is not something users will (or should have to) do.
That's a terrible idea. Menus shouldn't intentionally be designed to be user hostile. Instead just give a warning message, preferably one that is actually informative and appropriately scary looking.
There is a middle ground between letting any malware on the machine install whatever extensions it wants with no user consent, and banning users from installing extensions entirely.