GMX Mail does not work in Firefox?

For some months now I have had the problem that I can no longer log into GMX Mail in the Mozilla Firefox browser of my laptop.

It comes out every time that I use an outdated browser, even though Firefox is always up to date.

Does anyone have the same problem or ideas I can still try?

I have no idea why that could be.

Causes Research:

1) Use another browser, e.g.
https://www.chip.de/...20393.html

2) Try another computer…

This is usually due to browser add-ons. This can happen if they are badly programmed or interfere with the website so much that it does not work properly anymore.

Often these are any "security" add-ons or anonymizers. These are changing websites so much that they produce mistakes. I would try to start Firefox without add-ons and see if it works and then gradually re-enable the add-ons.