Just to help the other volunteers: Ctrl is Strg on German keyboards.

Does it work in other applications? Is the issue a global one in Firefox, or it affects specific sites?

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

no; the problem still exists. i'm working with the web interface by gmx-mail. In the past it was working. Probably one of the updates from Version 86.x

Stopped working for me on FireFox Developer 88.

Stopped working for me on FireFox Developer 88.

This is also on GMX webmail?

Have you modified the setting that allows websites to observe cut/copy/paste events with their scripts? You can check that here:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste dom.event.clipboardevents.enabled and pause while the list is filtered

(3) If the preference is bolded and has a value of false, double-click it to restore the default value of true

If the keyboard shortcuts still do not work with that preference on true, hmm, what have they done?!