I though I added a screenshot, here it is.

What font is Firefox using on these pages?

You can right-click and select "Inspect Element" to open the builtin Inspector with this element selected. You can check the font used for selected text in the Font tab in the right pane of the Inspector.

• https://developer.mozilla.org/en/Tools/Page_Inspector/How_to/View_fonts

Arial, sans-serif.

removing Arial from the stylesheet show the content. Wow, didnt think of that. So it must be my arial font that is corrupted?

Looks like Verdana too, Microsoft fonts - I'll see if I can reinstall those.

(Can I edit an answer? I can't seem to find the option)

Those fonts aren't installed in my computer.

Any update?

You can do a font test to see if you can identify corrupted font(s).

• http://browserspy.dk/fonts-flash.php?detail=1

You can try different default fonts and temporarily disable website fonts to test the selected default font.

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