Even without the web integrity api , the tests for amazon hiring process at least in india require you to run windows/mac and chrome, linux and firefox are not supported !
Some companies do that, it's not some new thing. We have to manage your devices and it's nice to standardize so we don't have to each learn every possible OS a user wants to install.
This usually means "you can use Firefox but don't ask support" instead of "you are not allowed to browse this website". If the latter applies, user agent switchers exist.