Thank you for sharing your Firefox add-on list. Could it be Adblock Plus blocking something needed by the slideshow script? You could try making an exception on the page and see whether it will work.

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
  remove checkmark: [ ] "Use hardware acceleration when available"
• https://support.mozilla.org/en-US/kb/performance-settings

Close and restart Firefox after modifying the setting to make the change effective.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

Disabling Ad Blocked had no effect. The photo gallery display the initial photo correctly with the advance return arrow. You click the advance and the page # changes to 2, the photo doesn't advance. The return click does nothing at this point. Doesn't go back to pg 1 or advance to pg 3

Firefox's Tracking Protection feature also may break some slide shows that wire each click to a tracking service. Do you see a shield icon toward the left end of the address bar, near the lock icon? More info on managing the Tracking Protection feature in this article: What happened to Tracking Protection?.

Tracking Protection was removed for this site and photos now advance. Thanks