Your web browser also has the ability to read and alter anything on any website, do you trust the makers of your web browser? (And everyone else in the supply chain...)
Not surprisingly it's easy for a organization to trust Mozilla Corporation than to trust a random Canadian guy.
I wouldn't call Raymond Hill a "random" Canadian guy. I am also fairly certain that these same organizations rely on code written by even more obscure, random individuals every day.
AFAIK Firefox add-ons go through review before being uploaded, so this would actually only be an issue if you don't trust both the Canadian guy and Mozilla.