I retested it and managed to activate the back button. I will close the issue as verified.
Bug 1719088 Comment 20 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
I retested it and managed to activate the back button. Aside from the latest Nightly, I also tested on the latest Beta (91.0.0-beta.6) where it also appears fixed. I will close the issue as verified.