Ironically though, I wonder if not using Firefox much is the cause of that annoyance. In other words, Firefox usually waits for an opportune time to update, but if it's been closed for a long time, it likely decides that it's too out of date, and updating asap is critical for security.

I'll reiterate my recommendation for ESR, it's Firefox but for people who want less frequent updates. :)

Firefox ESR updates on the same frequency as the mainline release, they're just more limited in scope (i.e. mostly just security & stability fixes).

They should be smaller downloads that install more quickly though, shouldn't they?

I'm not on Firefox ESR, but switching to Windows LTSC made Windows updates a lot less painful and less noticeable, even though technically the frequency is the same.

You can use ESR[0] version of firefox the equivalent of LTSB windows. Or use a registry hack[1] to disable updates for regular firefox if you use Windows.

0. https://ftp.mozilla.org/pub/firefox/releases/68.8.0esr/ 1. https://winaero.com/blog/disable-updates-firefox-63-above/

No, they just update. Leaving you dead in the water when stuff goes wrong.