Do you use a bookmark or are you starting with the main (home) page of this website?

If you use a bookmark to access a specific page, instead navigate to this page starting with the main page or with the sign in page in case there is a problem with this bookmark.

You can remove all data stored in Firefox for a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be careful. If you have a password or other data for that domain that you do not want to lose, make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website, data for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

Followed all directions and still nothing. I get the very same message and cannot play the game.

Firefox should switch to HTML5 like WebAssembly (WASM). Any relevant-looking messages in the Web Console?

• "3-bar" menu button -> Web Developer or Tools -> Browser Tools -> Web Developer Tools
• https://firefox-source-docs.mozilla.org/devtools-user/web_console/

Sorry but you've lost me. I haven't a clue as to what or where I should be looking once I open Web Developer Tools. But I see nothing referencing HTML5

Can you post the link to the site so others can see what happens so they know what to look for or if there is issue with the site itself?

Well I can but it is via MY Facebook sign in so I don't know that it would work for you. It's available only through Facebook. If you have a Facebook membership then I would suggest you try it with your membership because I am not giving out mine.