A custom hotkey might be overriding the default one in Firefox. Go to "System Preferences", then click "Keyboard", then go to the "Keyboard Shortcuts" tab. Select the "Application Shortcuts" item in the sidebar, and check if "Show Preferences" or something similar is listed. If so, delete it.

NOPE.

What exactly are you pressing? Your title says you are pressing "CMD-" (the hotkey for zooming out.) Your explanation says you are pressing "CMD+" (the hotkey for zooming in.)

I wrote it two different ways, but isn't it obvious? CMD + comma. It's the Preferences shortcut, and it's causing an un-zoomable zoom instead of displaying Preferences.

You could use this addon to fix your problem. Install it, then go to the "Preferences" window. Select the "Shortcuts" tab, and scroll down until you see the "Preferences" item. Change it to "Meta+,".

hey iNerd instead of guessing at solutions why don't you STOP

I tested it, and it worked... I wasn't guessing.

Either I'm not explaining the issue well or you're not getting it

I'm going to refer someone else here to help you. I am new to this :(

Sorry for the delay in responding; I was gone this weekend. I have contacted another assistant, and he said to create another profile and see if that fixes the problem.

See https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles