Firefox will use HTML by default, (unless; I guess; if the website tries to force it to do otherwise)

The Flash Player will already be set to ask to activate. You could even routinely set it to disabled and only enable Flash Player when you specifically need it.

I was going to post screenshots but this profile has broken again. Presume some odd problem with my setup and profiles - Sync inadvertently somehow activated I do not think that helped. I will try to get that fixed before answering more questions. No reason at present to suspect my issue is a Firefox bug.

Update,

• Happy112 : Well done for finding that add-on
• My profile issue may simply have been my impatience and the Compatibility Reporter https://addons.mozilla.org/firefox/addon/add-on-compatibility-reporter taking a long time to populate the extensions listing in that particular profile

Hi   !
'Found this add-on that you might like:
https://addons.mozilla.org/en-US/firefox//addon/html5-video-everywhere/

Did you also note that HTML5 Video Everywhere! 0.3.4 mentions:

Note: Some legacy sites force using Flash player, you will need to install no-flash add-on to support them.