Reset the page zoom on pages that cause problems.

• View > Zoom > Reset (Ctrl+0 (zero); Cmd+0 on Mac)

• http://kb.mozillazine.org/Zoom_text_of_web_pages

Start Firefox in Safe Mode 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).

• Do NOT click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode

Try to disable hardware acceleration in Firefox.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

• https://hacks.mozilla.org/2010/09/hardware-acceleration/

Same here i took a screeshot. Also my friend is having this issue too.

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

Thanks guys (& gals, if applicable). I found the "Zynga Game bar" add-on was causing the issue...