You're not really giving the whole story here.

A lot of people slapped a 'Don't use IE, it's old and broken message" on their sites because it was old and broken. Generally this didn't show for newer versions – though you're right in that a minor number of 'douch-baggy' sites did.

It was a significant and in some cases crippling workload to support end-of-life browsers that were lacking features such as e.g. transparent image support. Telling users to upgrade was a perfectly sensible thing to do in these cases – if it was suitable message for your customer base.

These days the situation is very different. There's almost definitely no way that the Play store doesn't work on Firefox. It's almost certainly being deliberately excluded from accessing the Play store for what can only be described as malicious reasons.