Thing is - that changed after FF40 update so it's Firefox's issue. Everything still works like a charm on FF 39.0.3.

I found a Windows XP system and after about an hour of updates and testing, I think I have a workaround. Which is, lie to YouTube and tell the site you run Firefox 39.

I started by installing Firefox 39.0.3 and got the first screen shot attached. I don't have H.264 support, which I think is the default state for XP. YouTube serves the Flash player.

I upgraded to Firefox 40.0.2 and got the second screen shot attached. YouTube now defaults to HTML5 and WebM, which only offers 360p.

I installed the extension https://addons.mozilla.org/firefox/addon/ua-site-switch/ which admittedly has a confusing interface. I opened its dialog on YouTube and entered this for the user agent:

Mozilla/5.0 (Windows NT 5.1; rv:39.0) Gecko/20100101 Firefox/39.0

Then I launched a private window (so as to avoid any influence of cookies YouTube/Google might have set already) and tested the html5 page again. Back to Firefox 39 behavior, as shown in the third screen shot.

Restarted in Safe Mode (extensions disabled) and opened a video: HTML5 player

Restart in normal mode (extensions enabled) and opened a video: Flash player

Missed something: embedded YouTube videos often launch from a different domain, so you need to apply the same lie to this site as well:

https://www.youtube-nocookie.com/

Walkaround? For such basic thing, something that should work fawlessly by default? I haven't tried it but what about other sites, do you have to set ua-site-switchseparately in all of them? It's a Firefox's issue and I'm going to wait until it's fixed.

Hi penguino, what this workaround demonstrates is that the change in Firefox 40 which is triggering the change on YouTube is just the version number. I'm pretty sure Mozilla isn't going to issue a new version of Firefox 40 with the version number 39.

If you prefer an older version of Firefox, consider the Extended Service Release version. At least that gets security updates. http://www.mozilla.org/firefox/organizations/

You don't think Mozilla is going to fix it? I thought they want Firefox to work properly.

"Extended Service Release" clearly says it's not for me: "Who is it not for? Individual users who always want the latest features"

Hi penguino, we are not communicating effectively.

What my test demonstrated is that there is nothing to fix in Firefox 40 to get back to using the Flash player on YouTube for Windows XP users. You just have to change the version number it reports to YouTube from 40 to 39. Boom => Flash.

This is down to how YouTube chooses to structure its pages based on your browser identification. It's not fixable by Mozilla.

So youtube trolled FF users and is now enforcing html player?

PS. I'm going to skip FF40 anyway, it was slower and there was more stuttering than on FF39 which is very smooth and fast, at least on my computer.