I suspect this may be due to the package format Firefox is installed as. By default Firefox is installed as a Flatpak, which is a containerized format with limited access to the filesystem. This would explain why it can't read the available printers.
I forgot to mention, you can check this really quickly by running the following command in the terminal (search for "terminal" in the application list):
flatpak list
If you see Firefox there, then it's installed as a Flatpak and you can replace it. Otherwise, it's most likely a different issue.
At least, this has been the case until very recently. The Zorin developers have decided to use the standard format for Debian-based distributions. I would recommend to install Firefox using this package format instead, following the instructions provided by Mozilla.
Please note that you can have have both versions of Firefox installed if you want. This can be helpful to quickly test if this is the actual cause for this particular issue without loosing your settings, bookmarks, etc.
But it can be quite tedious to "guess" which version of Firefox you're running at any given time so it'd be better to keep one or the other.
Here's a step by step post to uninstall Flatpak and install the .deb version: