If it’s default behaviour for static pages, but not for dynamic pages, then it’s not much of a default. As a user I want ui actions to be consistent, the unknown stuff that happens in the background shouldn’t change the behaviour of the ui. Firefox now mostly gets around this issue by opening search engine links in new tabs (I can’t recall if that’s standard for Firefox or if I had to change settings, but I’ve been using it for years like that), but this wouldn’t have been needed if using the back button was reliable.
If it’s default behaviour for static pages, but not for dynamic pages, then it’s not much of a default. As a user I want ui actions to be consistent, the unknown stuff that happens in the background shouldn’t change the behaviour of the ui. Firefox now mostly gets around this issue by opening search engine links in new tabs (I can’t recall if that’s standard for Firefox or if I had to change settings, but I’ve been using it for years like that), but this wouldn’t have been needed if using the back button was reliable.
I found this 2022-2024 discussion with a few examples of the back button not working as expected: https://connect.mozilla.org/t5/discussions/firefox-needs-to-do-something-about-back-button-history-loading/td-p/4678
I found no announcements of a fix. I also haven’t had much occasion of noticing a fix, since new pages open in a separate tab for me.