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

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

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

Thank you, Cor-el, for posting so quickly. Restarting in Safe Mode cured the problem, but I want to be able to use my add-ons. There were no corrupted fonts and disabling website fonts when using FF in normal mode didn't cure it. I assume therefore that it is one of my add-ons which is causing the problem.

How can I determine which of the add-ons is causing this?

I disabled Foxreplace, which was causing the problem.

Edit: this post should have been in the other thread about this issue ([*]).

<strike>Yes, this issue recently came up in this thread:

• [/questions/931527]</strike>

Try to install the FoxReplace 0.3.11 version:

• https://addons.mozilla.org/firefox/addon/foxreplace/versions/0.13.1