I'm with you, but I do think the situation can be characterized differently in a couple important ways:
1. IE was the default browser for many users (i.e. anybody using Windows who didn't know better).
2. IE had a lot of bugs and and was often non-compliant with standards.
Those two things combined meant that supporting IE required additional work, and if you didn't put in that work you were going to get users from IE anyway they'd just get frustrated and confused when things broke. So "detect IE and tell them use something else" was at least a reasonable fixed-cost approach to not having users get totally stuck. (And IE went down to 2-3% at least in part because devs revolted against IE earlier and started serving those "don't use IE" messages when its usage was still higher.)
Neither factor is really true of FF. It's not the default for any major platform, its user-base at this point is largely power users who won't be easily confused, and outside of some non-standard APIs most sites don't need and some fairly edge-casey stuff, most sites that work on Chrome will work fine on FF as well without alteration. If anything, IME Safari is more likely to need special attention than FF (but of course Safari has much higher market share so it merits that effort).
So I totally get not wanting to spend QA budget on FF, and I could understand showing a small banner suggesting you use a different browser, but erroring/completely blocking usage of the site does feel excessive to me, and even a bit mean-spirited since it takes extra effort to detect FF to show the message and prevent using the site! I don't think these sites are going out of their way to block usage of other low-usage browsers (some of which can alter behavior that could break some sites even if they are Chromium-based).