This can be a problem with the font that is used to display the text.

You can use this extension to see which fonts are used for text that is selected (right-click context menu: Show fonts in selection).

• fontinfo: https://addons.mozilla.org/firefox/addon/fontinfo/

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.

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

• https://support.mozilla.org/kb/Options+window+-+Content+panel

Thank yoy very much. For the moment it has been solved by disabling website fonts on firefox. Hope will continue working.

Thanks

PD. Here some images of the problem