Usually when I see this from non-spam sites, it's not even pushstate, it's just some page that redirects as soon as it loads. So you press back twice and it goes back -> forwards -> back -> forwards. Disabling pushstate doesn't fix that, it just makes pushstate equivalent to a redirect.

Single Page Applications use the History API to create a working back/forward history within the SPA. This will cause you to navigate away on use, and potentially lose data.

Browser.history.allowPushState was deprecated in Firefox V47 (2016) once they'd restricted the ability of sites to muck with history state via JS. This used to be a pretty big issue but as a daily Firefox user for 20 years, sites changing history state hasn't been a problem in recent memory, so whatever they did works pretty well.

But the TFA is about a related issue with a similar symptom, hijacking (or disabling) the back button in Chrome browser. This also hasn't been an issue in Firefox in recent memory (kind of shocked to learn it still was until now in Chrome). However, I did have a problem in recent years in Firefox with sites hijacking the Browser_Back keycode and/or hotkey (keycode 166 or Alt+Left Arrow) but I solved it with a small UserScript I posted elsewhere in this thread.

I rarely click the back arrow icon on the interface since I have a three-finger tap on my touchpad assigned to send the Browser_Back keycode when the active window is Firefox. Being a keycode, this can be intercepted by site JS. While sites intercepting Browser_Back (keycode 166) is rare, some video players use the arrow keys to skip forward/back and Alt+Arrow to skip more. Since Firefox uses Alt+Left Arrow as the back hotkey, this can be an issue. I fixed it with a UserScript that prevents sites from blocking certain keycodes. Note: you can also change all Firefox's hotkeys by going to "about:keyboard".