I had hopes for this, Mozilla seemingly killed the ability to save zoom levels since FF60, but there are too many bugs in this add-on.

I have to constantly reset every time I leave a page. As an example I use a home page that is an RSS reader, it becomes a mess, the main page reverts to some setting that I never used, the links within are a complete mess, they just randomly choose a zoom level.

There is no way to add custom settings, the Manage Rules setting is greyed out, nothing can be saved.

Too bad.

Update: I have checked "enable" many times both under options in my add-ons list and using the icon on the toolbar (or whatever it's now called), it doesn't matter, it won't save.

I will keep monitoring your updates and try them out periodically. I don't normally write this much about an add-on, but I do think you're on the right track and I need something like this that fixes the Mozilla mess on something as simple as zoom.