It's insane how much access companies give MS. The insights they must have on so many companies by snooping on what they do in outlook and excel alone...
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...)
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.
ZScaler comes from a vendor and a purchase agreement, uBlock Origin comes from "some guy" for free and that makes corporate folks uneasy. If Firefox would offer uBlock Origin in their enterprise version for $100/seat/month and market it as an "endpoint malware blocker for the cloud" instead of an ad blocker they'd probably get a lot of uptake.