Strange Grey Font when using Firefox

There is sometimes a strange grey font in Firefox. How can i fix that?

I can’t upload images :frowning:

Here’s a Screenshot
https://cdn.tixte.com/uploads/byzero.steals-code.tk/kqwa0l9zk9a.png

Have you checked this?

https://support.mozilla.org/en-US/kb/change-fonts-and-colors-websites-use

Oh, thanks