As I already stated, I wanted to know if any other users experienced the same.
I totally understand that browsers have started to prevent autoplay for music. The question is now, "why do my old files still work that were created in C2". Not only do they still play startup music fine in Firefox, but the same files work in C3 desktop too!
Additionaly, C3 allows you to use 'on Start' to play music still, and it works in Firefox no problem.
A work around is to preload the music file by placing it in the Sounds folder instead of the Music folder - no user input required. Although, yes I do know that it will be loaded into memory which may not be desirable for some.
For this topic an example file isn't required, otherwise I would have included one.
I'm sorry that I enquired now.