Do many companies actually do this though? I've only ever been with companies that test against Chrome. The one time at FF bug was found, that company told that client to switch to Chrome.
I know there was a kerfuffle about Airbnb and Groupon only doing Chrome as well.
https://www.reddit.com/r/AirBnB/comments/dv7v6n/customer_sup...
https://thenextweb.com/dd/2017/11/28/please-build-websites-w...
Maybe in theory all platforms are considered, but in practice many companies just do one and the others mostly just work.