Could you visit these two test pages and tell me the results:

(1) The YouTube HTML5 test page: do you have all blue boxes except the last one? If you have other red boxes, which ones are they?

https://www.youtube.com/html5

(2) The H.264/MP4 tester (scroll down past the text to where the players are supposed to appear):

http://www.quirksmode.org/html5/tests/video.html

Do any of those work?

By the way, an update is available to your version of Firefox. You can get it using the "About Firefox" dialog as described in this article: Update Firefox to the latest release.

Thanks for your reply.

To answer your questions:

(1) On the YouTube HTML5 test page I have all blue boxes except for the second (H.264) and fifth (MSE & H.264).

(2) On the H.264/MP4 tester the first player (H.264/MP4) does not work (it says "No video with supported format and MME type found") but the other two (WebM & Ogg/Theora) do.

Following your advice, I used the "About Firefox" dialogue to update my version of FF, but that has made no difference to this.

I don't know if this will help with diagnosing the problem but I should perhaps add that when I try to play these videos I get a popup box asking me which program I want to open the video with, the options being "VLC.MP4 (default)", "Media Center" or "Other". Obviously, I don't want to open it with any special program in a new window, but rather simply stream it where it is.

Thanks in advance.

Hmm, usually on Vista Firefox can use the Windows Media Foundation to decode H.264 video.

Do you have an "unbundled" version of Windows that needs media components installed separately? You could try the link on this page under Vista: http://windows.microsoft.com/en-us/windows/download-windows-media-player.

Could you also check these preferences:

(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 wmf and pause while the list is filtered

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

(4) In the search box above the list, replace wmf with mp4 and pause while the list is filtered

(5) If either media.mp4.enabled or media.mediasource.mp4.enabled is bolded and "user set" to false, double-click it to restore the default value of true

If you reload the test pages, does that make any difference?

Many thanks again for your reply.

I'm afraid I have no idea whether my version of Windows is "unbundled", but I tried to download Windows Player 11 from the link you provided and got this message:

Windows Media Player is not compatible with your system. You are running Windows Vista 32-bit. Although Windows Media Player will not run on your system, you can download Windows Media Player for other operating systems.

I also went through your steps (1) to (5) and all the preferences you cite were already unbolded and set to true.

Many thanks for your persistence anyway, and sorry it's not as straightforward as we might have hoped.

Hi Damian_V, I think you should try:

Windows Media Player 11 for Windows XP

on this page: http://go.microsoft.com/fwlink/p/?LinkId=158425

Thanks, I have tried that and it says I already have a more recent version of Windows Media Player on my computer and so will not allow me to proceed. When I try to play a video and am given the popup box mentioned in my earlier comments I am able to select Windows Media Player, but when I do and the Media Player opens I get another popup box saying "The selected file has an extension (.mp4) that is not recognized by Windows Media Player". It gives me the option of trying to play it anyway, but when I select this I get a popup saying "Windows Media Player cannot play the file. The Player might not support the file type or might not support the codec that was used to compress the file". Underneath this appear two buttons, one for "Close" and one for "Web Help". When I select the latter it takes me here: http://windows.microsoft.com/en-gb/windows7/c00d1199

I have not looked into the whole "codecs" thing, however, as I was assuming (perhaps wrongly) that I shouldn't need Windows Player to start up in order to play a video, but that it should play on the website itself as it would on e.g. Google Chrome.

Again, many thanks for your persistence.

Could you check whether graphics card/chipset driver updates are available for your system. This article suggests safe ways to do that: Upgrade your graphics drivers to use hardware acceleration and WebGL. I think that could be the main issue, if your Vista is fully updated.

I don't know why the following wasn't linked in that other article:

Media foundation

This update enables the playback of MP4 files, H.264 files, and AAC files by using the Source Reader component. Because of this improvement, a Windows compatible browser can handle contents that include the video tag of HTML 5 correctly.

Platform Update Supplement for Windows Vista and for Windows Server 2008

That's actually from 2011. Can you tell whether you already have it?

I followed the first link you provided and determined that there were five "important" and three "optional" updates. I initially set it to install all of these updates, but after installing the first it got stuck on the next for several hours. Since I needed to use the computer I then stopped it and had to restart the computer. Having done so, with only one update installed, things seem a little slower than before, and I'm concerned that installing all eight of these new updates will slow things down still further. For these reasons, I think I'm best off just going back into Google Chrome when I want to watch those videos (it's only that IaI TV channel I've had problems with so far in any case, and please do correct me if I'm wrong but I don't think there'd be too much risk using Chrome, even with Vista, for such things now and again). If you think my worries about Windows updates are unfounded, I'll be happy to take your advice and try again, but I really don't think this is important enough for me to risk dramatically slowing down the computer's general performance. If you agree, I'll leave it there, and hopefully this page will still prove useful for others, as some of the solutions you've recommended above are bound to work for some cases.

Many thanks again; your help is greatly appreciated.

Hi Damian_V, there are a lot of shades of gray in "slow" and "risk" so it's difficult to make a judgment for you. Some "important" Windows updates may be security-related, so I think you would still want to install those.

Hi jscher2000. A good point well made about the many shades of grey and trade-offs regarding degrees of processing speed and risk. Since you advise that I ought to install the "important" windows updates, I will do so. If things seem okay after that, I'll also install the "optional" ones and see if that makes any difference. I will be sure to report on it here.

Regarding the second part of your 4/8/16, 4:35 PM reply you asked "Can you tell whether you already have it?" Apologies for my ignorance, but would you be so kind as to tell me how I might determine that?

Many thanks in advance.

Hi jscher2000.

Just an update, as promised in my previous comment: I haved now installed all the Windows updates, but unfortunately it has not solved the problem.

I guess the second part of your of your 4/8/16, 4:35 PM reply might be worth pursuing (please the second paragraph of my last comment from 9am yesterday) but no worries if you've lost patience with this now.

Many thanks again for all your help and advice.

Hi Damian_V, I'm not sure about Vista, but on Windows 7, you can check whether a Windows update has been applied using:

start menu > Control Panel > Uninstall a Program > (left column) Installed Updates

There is a KB number in parentheses, and that platform update is KB2117917.

Hi jscher2000. Thanks, I followed your instructions and, as far as I could tell, I did not already have the update, so attempted to install Platform Update Supplement for Windows Vista (KB2117917). Unfortunately, however, I received a popup saying "The update does not apply to your system". Anyway, I've pretty much resigned myself to having to open a different browser to play such videos now, but many thanks once again for all your help and advice. Hopefully it will prove useful to others (as it has to me, at least in terms of knowing my way around my computer a little more, and knowing how to check for/install updates and such like).