If you are not able to use HTML5 player on Youtube then you may be able to select using Flash Player on https://www.youtube.com/html5/

Note Firefox 39.0 is EOL and vulnerable as it does not have the security fixes that Firefox 50.0.2 and 45.5.1esr has.

The EOL WinXP is not able to use HTML5 without some workarounds as Microsoft does not provide the necessary codecs for WInXP but does for Vista and later.

YouTube Flash Video Player:

• https://addons.mozilla.org/en-US/firefox/addon/youtube-flash-video-player/

@James I cannot select Flash Player. First - I was talking about switching UA agent to FF39 because Youtube used to use Flash for older versions of this browser. It seems it's no longer a thing. Second - I have performance and stability issues with every Firefox version above 39.0.3. They were optimized for newer devices with full HTML5 support.

@cor-el That's the one I'm using right now. Transition from HTML5 to Flash is very visible and lasts around 2 seconds (html player loads first, for a split second there's an information that it cannot play media and than flash player starts to load). Once it crashed one of the drivers and I had to restart my computer. It's not very convenient so I'd like to know if there's any way to play youtube videos in flash by default.

Can't you simply disable your html5 player ?

Firefox/Tools/Add-ons/Plugins Set HTML player on never activate.

If you don't have the html player among your plugins, you could try to install it and then disable it in the above way. Perhaps shutting that html player off will also shut the default html player off. And fox will then automatically use your flash player since that is the only other tool that can playback the video then :)

Good luck

Hi Passie, the HTML5 player is a built-in feature and isn't listed on the Add-ons page.

Hi penguino, you can turn off the HTML5 video player on YouTube using the following settings, but on other sites that do not switch to Flash with these settings, you will not be able to view the videos. (I suppose that problem exists anyway since most such sites will be using MP4, which Firefox cannot decode on IE.)

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste mp4 and pause while the list is filtered

(3) Double-click the media.mediasource.mp4.enabled preference to switch the value from true to false

(4) Double-click the media.mp4.enabled preference to switch the value from true to false

(5) In the search box above the list, type or paste webm and pause while the list is filtered

(6) If the media.mediasource.webm.enabled preference is bolded and "user set" to false true, double-click it to restore the default value of true false

(7) Double-click the media.webm.enabled preference to switch the value from true to false

YouTube's scripts will check whether you can play MP4 or WebM, briefly display the error/static image, then switch to Flash. No extension needed.