Some more info. using 91.6.1esr now. When one such tab refuses to work, I move it to a dedicated FF window which I titled "BAD". After a while, this tab crashes, a report is automatically submitted, I click restart all crashed tabs, and they do, AFAIK. This is the last one: https://crash-stats.mozilla.org/report/index/2ba1923f-2d66-4997-9c84-34eef0220310