Pages won't go back to the last page either with the back button or mouse button in Firefox for me.
I did notice once you go to a page and immediately hit the back button, it will go back no issues. If you wait a few seconds instead, then the back button will be broken.
It's due to some kind of script that's loading. Once it loads, the back button jumps to the bottom of the page or becomes useless. I believe it's possibly related to ad blocking. Though, I also have adblockers on Chrome and Chrome is just fine.