Such choices are saved as part of the Site Preferences.

In case you use "Clear history when Firefox closes" or otherwise clear history.

• do not clear the Site Preferences

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exception for password and other website specific data
• clearing "Cookies" will remove all selected cookies including cookies with an "Allow" exception you may want to keep

Hello helpneeded,

Would you do this please :

When you're on that site, click on the 'i' in a circle, next to the padlock at the left side of the address bar.

Then click on the little black arrow on the dropdown and then on "More Information", which will take you to "Page Info".

Click on the 'Permissions" tab and scroll down to "Run Adobe Flash". Uncheck "Use Default" and check "Allow".

Then close and restart Firefox.

In addition :

It might also help to set Shockwave Flash to "Always Activate" in 3-bar menu (or Tools) => Add-ons => Plugins.