1st. Clear Firefox cache and data

    Menu → History → Clear Recent History...
Restart Firefox and re-visit Wikipedia.

if the problem still exists try

2nd. Try Firefox's Font Settings You can force Firefox to ignore the website's specified fonts to confirm if is font issue

   Menu → Settings → General.
   Go to Language and Appearance → Fonts & Colors → click Advanced...
   Uncheck the box that says "Allow pages to choose their own fonts, instead of my selections above."
   Click OK and reload the Wikipedia page.

If this fixes the issue, it point a local font problem. You should re-enable this option after that.

These solutions didn't help, sorry. I cleaned history "since 00:00"

Ok If 1 & 2 didn't help you, It doesn't seem to be a problem with Firefox, but with your system.

You need to figure out exactly which problematic font is being used.

Go to a Webpage that shows that characters. Right-click on the wrong letter text and select Inspect. This opens the Firefox Developer Tools.

In that panel, look for the Fonts tab. The Fonts tab will show you the exact font file being used by Firefox. Identify the font name.

You need to uninstall/reinstall that font.

On Windows: Go to Control Panel → Appearance and Personalization → Fonts (or search for "Fonts" in the Start Menu). Find the problematic font (e.g., Arial ) and Uninstall.

Restart your computer.

If the font is a standard system font, Windows may automatically reinstall a clean copy on restart, or you may need to find and download a fresh, clean Font version to reinstall.

It is indeed Arial for me but Windows wont let me delete it. "Protected system font"

You can try the system file checker (cmd > sfc /scannow) can scan and repair it

or

try to download Arial font again and re-install