Start Firefox in Safe Mode to Troubleshoot the issue and to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Do you have that problem on other browsers too?

What font is used in those cases?

• TynineSansMono Reg Regular: Used as: "TyponineSans Monospace Regular 4"

You can right-click on a web page and select "Inspect Element" to open the Inspector (Firefox/Tools > Web Developer). You can check the font used for selected text in the Font tab in the right pane of the Inspector.

• https://developer.mozilla.org/Tools/Page_Inspector

Make sure that you allow pages to choose their own fonts.

• Tools > Options > Content : Fonts & Colors > Advanced: [X] "Allow pages to choose their own fonts, instead of my selections above"

There was no problems displaying the website in IE.

I tried to start Firefox in safemode, and the problem was gone. Then I started Firefox normally and the problem now seem to be gone. Very strange...

Try to disable the addons one by one and check, which addon making this issue.