Same here. This is only on one laptop that I use for rubbish but the full reload takes a few minutes.

Might be OS dependant? My Firefox does not reload the page, it's just "there". I have to refresh to get an update of the page (I use it for forums).

I'm on Windows if that makes any difference.

I think it rather depends on the page (some pages with lots of dynamically generated content might mark themselves as no-cache/no-store), and in any case if you don't use a tab long enough, sooner or later it'll be evicted from the browser cache.

Maybe, just running on bog standard Linux Mint. Nothing fancy.

cache_control http header of the site

No. Every site, even HN and static sites.

Remember, we’re taking about a Firefox restart, not the back button.

Sites like Facebook or Reddit do indeed do a full reload, but most sites I frequent do not. Nice to be able to pick up where I left. If I want updated content I just hit F5.

We’re talking about coming from a restart, not the back button.

Yes, that's what I was talking about too. Like I said I have to hit F5 to get updated content after a restart on most sites.