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

See also:

• https://support.mozilla.org/kb/Websites+look+wrong

Can you attach a screenshot?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Use a compressed image type like PNG or JPG to save the screenshot.

OK - Starting in safe mode does appear to remove the problem

However if I manually disable (and then restart FF) all my add ons then the problem persists.

This would indicate that it's not an add-on problem but something else.

Safe mode appears to leave all plugins enabled - could it be one of these?

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/

In Firefox Safe mode these changes are effective:

• all extensions are disabled
• the default theme is used (no persona)
• userChrome.css and userContent.css are ignored
• the default toolbar layout is used (localstore-safe.rdf)
• the Javascript JIT compiler is disabled
• hardware acceleration is disabled

• plugins are not affected
• preferences are not affected