Hi,

You can try to Clear Now the Cache, Cookies, and Site Preferences in Tools (Alt + T) > Clear Recent History. If the problem persists, please check if this happens in Safe Mode. Safe mode disables the installed Extensions, and themes (Appearance) in Tools (Alt + T) > Add-ons. Hardware acceleration is also temporarily disabled - the manual setting is Tools > Options > Advanced > General > Use hardware acceleration when available. All these settings/options/add-ons can also be individually or collectively disabled/enabled/changed in Firefox normal mode to check if an extension, theme, option or hardware acceleration is causing issues. Disabling/enabling hardware acceleration, and some types of add-ons in normal mode may require a Firefox restart.

Uninstalling Add-ons

Uninstalling Toolbars

Troubleshooting Extensions and Themes

Problematic Extensions

Options

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

• Don't make any changes on the Safe mode start window.
• 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://hacks.mozilla.org/2010/09/hardware-acceleration/

This can also 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 selected text.

• 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 disable website fonts as a test.

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

• https://support.mozilla.org/kb/Changing+fonts+and+colors