@Gargron Well, I don't know about "outdated", but it's the same version numbering as every other OS.

My problem is that the version of Firefox I'm running on my MacBook Pro loads Mastodon just fine until every once in a while, it gags and coughs up the error.

Perhaps you have a bug in your code, as the message says. The "report" link taking me to GitHub means absolute zero to me, as I'm not a programmer.