Earlier quoted context omitted.
This is a nice sentiment but for actual products you end up having to say X versions of Y browsers are supported. It ends up being a feature requested by customer support teams that you prevent the app from working on unsupported browsers because users don’t read banners or pop ups and it generates pointless tickets. If you make your users change their UA there’s an awareness that things might break.
'unsupported browsers' are browsers that are old or broken. FireFox is a perfectly good browser, it is current and should be supported.
'supported browsers' are browsers that you lose customers if you don't support. You don't lose customers by telling them to switch to Chrome. Firefox's market share is small and the number of people who exclusively use Firefox and refuse to switch is vanishingly small.