Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: "3-bar" menu button or Tools > Add-ons > Appearance
• Do NOT click the "Refresh Firefox" button on the Safe Mode start window

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

Thanks for your reply. I think I've already tried all that (unless I've mis-understood). I'm using default theme normally, but this shows as disabled in Safe Mode.

In Safe Mode the Browser Console (Ctrl Shift+J) is still showing the continuous error.

Also in Safe Mode the Health Report shows three extensions as active. I thought they'd all be disbled. Is there any way to find out which they are (and where they're located)? In normal mode I can only see seven enabled extensions in Add-ons Manager (and several disabled ones) but Health Report says ten. All plugi-ins are currently on 'Never Activate'.

LATER: After I'd made this post I closed Firefox. The Browser Console was still open and it has continued to clock up the error (about one per second). I don't know enough about how theses things work, but could it be the Console itself that's the problem? Against that idea: 1. Many of these errors are already logged when I open the Consiole. 2. The same thing happens in both the 'Browser Console' and the 'Error Console' (although I assume they are closely related).

One other point: Firefox Developer Edition (48.0a2, dated 24 May 2016) is NOT showing this problem. I have the same Extensions there as on my regular Firefox.

Those three active extensions are probably the extensions that are part of built-in features.

• e10srollout: e10srollout@mozilla.org
• loop: loop@mozilla.org
• pocket: firefox@getpocket.com

Can you post your extensions list?

• https://support.mozilla.org/kb/Using+the+Troubleshooting+Information+page

Thank for your reply. I'd hoped to attach a file, but it looks as if I can only add images. So here's the list of Extensions that are enabled:

Classic Theme Restorer
Clear Cache Button
Custom Buttons
Firebug
*Firefox Hello Beta
Googlebar Lite
*Multi-process staged rollout
*Pocket
Saved Password Editor
Web Developer

Those with * are not in the Ad-ons list, so it looks as if you were right about their identities.

The error messages persist even when in Safe Mode, so I hope the Extensions can be eliminated. I have the same extensions in Firefox Dev version and it runs OK.