Is this issue exclusive to Firefox because this seems like a damaged or missing font file on your computer to me. Try the solution provided in [https://answers.microsoft.com/en-us/windows/forum/windows_7-desktop/how-do-i-fix-corrupted-font-files-in-windows-7/7860617d-e4a8-4792-a84b-b83f4bfd81b4 this Microsoft support thread] to ensure that all the fonts on your system are working properly.