Browser add-ons require permissions which you might not want to grant to arbitrary code. And anyway, it’s a truism that products can and will absorb add-ons into their code (cf ‘Sherlocked’)
If you don't trust, don't install. Maybe don't install anything on your machine just to be safe
Firefox can improve their addons security audit, their addons website to help users make a choice... absorbing addons seems like wasting time