I think that the issue is known, but hard to track. It shows with NVDA and heavy web apps like Twitter and Messenger if they are running for some time. I'm posting here some of the crash ids, but it is likely that they are not representative. When the browser starts to clock up, it locks the screen reader as well and I'm not waiting for the crash, but kill the process with extreme prejudice. I'd be glad to help in any other way.
bp-cb27fd07-ad27-49bf-b6c2-9dbbb0211005
bp-96eef135-da51-45a8-a792-d28810211005
bp-184a8916-227c-42c4-8308-6e3c00211005
bp-ba6748d3-2d93-4d84-992e-8cf960211005
bp-8bdfb00f-0e30-4cd3-826c-efb910211005
Here is the Bugzilla ticket: https://bugzilla.mozilla.org/show_bug.cgi?id=1572915