End of Flash and "Run Flash?" prompts in the browser

Based on the documentation at https://articulate.com/support/article/How-the-End-of-Flash-Affects-Articulate-Apps-and-Courses and within that post: "Your published courses will also continue to work if they include HTML5 output..." I thought I was clear about what needed to be done with with my published courses to prepare for the end of Flash. Based on this info from Articulate, everything I've published via Articulate 360 that includes HTML5 output will continue to work.

However, at the present time, in some browsers, I get a prompt "Run Flash?". I have seen this in Firefox browser in particular. This happens with courses that were published "Flash with HTML5 Fallback." In courses published with the reverse setting "HTML5 with Flash Fallback" I don't get this prompt.

I realize there are differences with how browsers function, but I'm wondering why the HTML5 fallback isn't happening seamlessly. If I need to republish courses that include HTML5 output, but with HTML5 as the first option "HTML5 with Flash Fallback" then I have a LOT of work to do.

I'm hoping someone from Articulate can comment on whether these publishing settings matter - is there any issue with whether Flash or HTML5 is the fallback? Thanks for any info on this!