XP can't handle HTML5 completely. If you want to use Flash at YouTube you'll need to install it. Firefox uses the Plugin version of Flash; IE uses the ActiveX version; Chrome uses the Pepper version of Flash.

Hi   !
First,   would you take a look at this:
www.theverge.com/2015/1/27/7926001/youtube-drops-flash-for-html5-video-default
There is an add-on that makes it possible for you to use Flash,   but you made it clear that you don't want extra add-ons   .....
But just in case you change your mind:
https://addons.mozilla.org/en-US/firefox/addon/youtube-flash-video-player/

First, as noted in the above reply, when the forum polled your plugins, "Shockwave Flash" was not detected, so that's the first problem.

Second, YouTube will check the formats your browser's HTML5 player can handle and will use Flash if neither MP4 nor WebM is supported. If you want to set Firefox to indicate that you can't play those formats -- this is not YouTube specific, it will affect all sites -- you can try this:

(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.

Hide support for MP4 formats -- Firefox probably can't play these on Windows XP anyway

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

(3) If the media.mediasource.mp4.enabled preference is currently set to true, double-click it to to switch the value from true to false

(4) If the media.mp4.enabled preference is currently set to true, double-click it to to switch the value from true to false

Hide support for WebM formats -- these are the typical "fallback" formats on YouTube

(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 currently set to true (it is false by default), double-click it to to switch the value from true to false

(7) If the media.webm.enabled preference is currently set to true, double-click it to to switch the value from true to false

(You can ignore the encoder and audio-related WebM preferences.)

Also, your Firefox identified itself to the forum as version 43. An update is available: this article describes how to get Firefox 49.0.1 using the "About Firefox" dialog on the help menu: Update Firefox to the latest release.

If something is holding you back from upgrading to Firefox 49, please let us know so we can suggest solutions or workarounds. Version 43 is not secure; Mozilla discloses security flaws after each new release.

Sometimes Firefox reports the wrong version because that information was saved in a preferences file. If the "About Firefox" dialog (see first article link) shows Firefox 49.0.1, you may need to clear out that incorrect information. See: How to reset the default user agent on Firefox.

The question is related to the latest version, but I installed an earlier version to se the effect that it had on the problem. Thanks for the help.

jscher2000 said

Also, your Firefox identified itself to the forum as version 43. An update is available: this article describes how to get Firefox 49.0.1 using the "About Firefox" dialog on the help menu: Update Firefox to the latest release. If something is holding you back from upgrading to Firefox 49, please let us know so we can suggest solutions or workarounds. Version 43 is not secure; Mozilla discloses security flaws after each new release. Sometimes Firefox reports the wrong version because that information was saved in a preferences file. If the "About Firefox" dialog (see first article link) shows Firefox 49.0.1, you may need to clear out that incorrect information. See: How to reset the default user agent on Firefox.

To see what your system support for HTML5 player https://www.youtube.com/html5/

The HTML5 player is currently used when possible on Youtube and may fallback to the Flash Player if you have the Flash Player Plugin (NPAPI) on system.

http://www.adobe.com/software/flash/about/ https://get.adobe.com/flashplayer/