Can you give an example link or two?

I'm also curious whether the fourth slide might have an ad that Adblock Plus is blocking. To try the slideshow with Adblock Plus sidelined, you could test those page in Firefox's Safe Mode. That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

Not all add-ons are disabled: Flash and other plugins still run

After Firefox shuts down, a small dialog should appear. Click "Start in Safe Mode" (not Reset).

Any difference?

Note that you should always check which items are blocked if you use extensions that can block content as you can never be sure what gets blocked.

You should be able to check that if you click the ABP toolbar icon.

That was precisely the problem! After disabling, slideshows now play through, including the lovely ads.

Thanks very much.