When I try to open play.google.com from Firefox on my Android device I get:
This browser is no longer supported.
Please use the Google Play Store app to access Google Play.
My Firefox version is 35.0, which is the latest version for Android. So it looks like the walls are going up around that particular garden as well.
And I'm sad seeing many of the arguments here - a wild guess is that they've been made by people who weren't here (or have forgotten) the Microsoft/IE story.
Soon it will become: "Why would they invest resources? Firefox is a niche browser, the return on investment is not justified". After all, Firefox's market share is dwindling.
I really hope we're not slowly being boiled like the proverbial frog, by Google.