Hello,

Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• You can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.

Thank you.

Thanks for the suggestion - I did mention in the OP that I had run without addons in FF24.0 and FF25.0. I'd initially formed the hypothesis that either FB or the specific game app was conflicting with adblock or that there was a Java issue, or another extension.
I was going to try another install on XP with a default about:config - but this all takes quite a bit of time, so I'm hoping to check if it's specific to my own installation before I spend even more time troubleshooting the issue.

***FIXED*** Just realised 3rd party cookies were disabled - changed to always visited and all is now well. How did I miss that?

Simon