@Gargron For the record, I'm posting this now from Firefox 87 on my MB Air running OSX 10.12.4 "Sierra".
Still doesn't explain why Mastodon runs fine under Firefox 48 under OSX 10.8.5 "Mountain Lion" except when it chokes at random when loading Mastodon every few months or so.
@flugennock Well, it's not like an app is either entirely compatible or entirely incompatible with a browser. Incompatible means it works until it encounters some unsupported function and then crashes.