Little page I missed from Firefox Roadmap for Flash End-of-Life
is the one pointing to
Flash Activation: Browser Comparison
on the Mozilla Developer Network
Each of the major browsers has now implemented a feature where Adobe Flash content does not run by default, but each of the browsers has a slightly different implementation and user interface. This guide will help outline the similarities and differences between browsers so that web developers can provide the best experience. Another guide is available to assist website authors in migration away from Flash.
In each browser, the decision to enable Flash is made by users on a per-site basis. When a site attempts to use Flash, the browser will prompt the user in some way and give the user an opportunity to enable Flash for that site.
Followed by screenshots in "UI Comparison" where you can see how different browsers do things well... differently.
Followed by some "Site Authoring Tips" etc.
Yeah indeed calling from Flash
flash.external.ExternalInterface API a JS callback
pluginCreated() is good advice.