Have you tried specifically blocking that pop-up and/or blocking all pop-ups? I realise you want to avoid having to do so but perhaps the settings aren't working as they should. (Exceptions don't always work.) You can block for specific sites with the Page Information dialogue. ctl+i, permissions tab > open pop-up windows.

I am not asking about blocking sites individually.

I am asking about the feature that is supposed to block all sites for all time.

And while I will probably visit The Daily Mail once a month or so, there are plenty of sites that I will visit once in 5 years. I do not want to see popups about push notifications.

Please note that when you had to activate this via about:config it actually worked.

I have blocked pop-up windows and that seems to work. If your objective is to have a defect in the software corrected, this is the wrong place to report it as most readers and posters here are ordinary users. You can file a bug report at https://bugzilla.mozilla.org/