Adding image.

Hi murphysmate, I haven't seen that before. I wonder whether that site is not accounting for the height of the toolbar and trying to squeeze the Flash content into too short a space, which seems to be corrected by zooming/resizing. Does it make any difference if you maximize the window?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

I tried maximising the window but it doesn't fix it. I'm going to try the safe mode thing now.

I've restarted in safe mode - now flash is disabled and the games won't work.

murphysmate said

I've restarted in safe mode - now flash is disabled and the games won't work.

That's strange, Flash should work in Firefox's Safe Mode, and the JavaScript routines often used to set up pages should work in Firefox's Safe Mode. Is there any error message?

Adobe also has a test page here:

https://helpx.adobe.com/flash-player.html

You may need to give permission to Adobe's site if you haven't before. See: Why do I have to click to activate plugins?

When I was in safe mode it had a message saying I needed to install flash. I don't get that when not in safe mode.

The animation on the test page works fine, and I have activated the plugin/given permission.

murphysmate said

When I was in safe mode it had a message saying I needed to install flash. I don't get that when not in safe mode.

That message might be caused by something blocking a script (e.g., ad blocker, script blocker, tracking blocker). If you don't think the issue is caused by an add-on, can you rule out the Tracking Protection feature? More info in this article: What happened to Tracking Protection??

I've turned off tracking protection, and the problem still happens.