Problem in Firefox in admin area

Dear supporters,

we are using WP Globus in a small wordpress installation with a really small and simple custom template created on our own.

Since a few months the editor of the page has the problem, that translations doesn’t work anymore in Firefox admin area. The visual view is empty in both languages, the code view in the main language contains the content in both languages with the {:de} shortcodes and in the second language is the correct content without the shortcodes.

I tried to deactivate all other plugins (except “Classic Editor and Classic Widgets”) but the problem still exists, even if I deactivate the last plugin for testing. In some of my many tries everything suddenly works fine again and the error seems to be gone, but after reactivating the plugins step by step (in some tries in random order) the problem reoccurs, but not after activating a special plugin. To my mind the problem isn’t connected to a special plugin.

Important: In Chrome everything is perfect and there is absolutely no problem at all!

In the web developer console is only one error, starting with:
Uncaught TypeError: n is null
https://domain.de/wp-includes/js/tinymce/wp-tinymce.js?ver=49110-20201110:3

To my mind it seems to be a bug in the plugin in connection with the Firefox browser. After setting up the blog in 2017 we didn’t add any plugins or did anything else than WP and plugin updates and suddenly the problem occured…

Sure, the editor just may change the browser, but I think it is in your interest to fix this bug (if it is really a bug).

If you need further information, let me know…

Best regards,
Christian