First, could you check your system preferences:

System Preferences => Keyboard => Shortcuts => Keyboard => "Move focus to next window"

Does that have the checkmark to enable it and show Command-Backtick as the shortcut?

On the Firefox side:

As far as I can tell, Firefox doesn't use that as a built-in keyboard shortcut: Keyboard shortcuts - Perform common Firefox tasks quickly.

You could check whether any of your add-ons is using it. See: Manage extension shortcuts in Firefox.

This feature is also known as "Command + ~" (tilde).

Can t write tilde or apostrophes or they appear without typing in any´page of Firefox, it´also happens in Chrome and Edge, but not in any other app´lication. In this line you can see my problem .’ for ‘Firefox