My SO just had something similar pop up yesterday. She was running into weird errors on her Chromebook, so I had her change her user agent to Chrome on Windows. Everything magically worked. Hmm…
What is the perceived problem, then? 99% of sites these days are all built with kits that support Firefox just as well as Chromium, the dev choice to not support or intentionally lock out Firefox is either just laziness (not wanting to deal with any potential problems or not given enough time to run full Firefox user tests) or incentive driven (middle manager has word from high manager that they can't support firefox because highest manager makes bank from Chromium).
The technical limitation isn't actually there in the modern web, it's almost always a manufactured limitation. I think I've only ever encountered a single website that didn't actually technically work on Firefox, and that was Weather Underground. Which they ended up fixing after 3 months or so.