Please check if all your plugins are up-to-date. To do this, go to the Mozilla Plugin Check site.

Once you're there, the site will check if all your plugins have the latest versions. If you see plugins in the list that have a yellow Update button or a red Update now button, please update these immediately.

To do so, please click each red or yellow button. Then you should see a site that allows you to download the latest version. Double-click the downloaded file to start the installation and follow the steps mentioned in the installation procedure.

Try disabling graphics hardware acceleration. Since this feature was added to Firefox, it has gradually improved, but there still are a few glitches.

You might need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.).

Then perform these steps:

• Click the orange Firefox button at the top left, then select the "Options" button, or, if there is no Firefox button at the top, go to Tools > Options.
• In the Firefox options window click the Advanced tab, then select "General".
• In the settings list, you should find the Use hardware acceleration when available checkbox. Uncheck this checkbox.
• Now, restart Firefox and see if the problems persist.

Additionally, please check for updates for your graphics driver by following the steps mentioned in the following Knowledge base articles:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

• Upgrade your graphics drivers to use hardware acceleration and WebGL

Did this fix your problems? Please report back soon.

@ComputerWhiz: checked my plugins, everything's up to date.

Disabled hardware acceleration in both Firefox and Flash options dialogs, no dice.

I doubt it's a driver-related issue as YouTube videos play without error in a different profile or browser.

When you say: "...YouTube videos play without error in a different profile or browser." Do you mean a different Firefox profile on that computer or a different Windows user on that computer?

Sorry, I should have been more clear.

On the same computer, I can play YouTube videos without error using IE or a different (clean) Firefox profile.

Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache. Note: This will temporarily log you out of all sites you're logged in to. To clear cache and cookies do the following:

1. Go to Firefox > History > Clear recent history or (if no Firefox button is shown) go to Tools > Clear recent history.
2. Under "Time range to clear", select "Everything".
3. Now, click the arrow next to Details to toggle the Details list active.
4. From the details list, check Cache and Cookies and uncheck everything else.
5. Now click the Clear now button.

Further information can be found in the Clear your cache, history and other personal information in Firefox article.

Did this fix your problems? Please report back soon.

I'm afraid that this is all that I can help you with tonight. We will resume tomorrow.
Sorry for any inconvenience.

Nope, cleared absolutely everything, still doesn't work.

Well, I can tell you this much: it has something to do with configuration.

I restored prefs.js from a clean Firefox profile to my regular profile and YouTube worked. Exactly what configuration variable made the difference, though, I have no idea.

Does anyone have some ideas to point me in the right direction?

OK, now I'm getting somewhere...

I disabled youtube.com in the HTTPS Everywhere rules and I can watch YouTube videos in this Firefox profile as long as it's not over SSL.

Would still like to know why it doesn't with with HTTPS though...

This had been happening to me too for ages and nothing made a difference... until I read a few lines on a forum somewhere that solved it in an instant...

YT started using Dynamic Adaptive Streaming over HTTP (dash) a few years ago and this is what's causing the problems. You can download a script called Youtube Center that allows you to turn off dash : https://github.com/YePpHa/YouTubeCenter/wiki

Or, as I found, you can simply plug in a set of headphones into your pc (or use a set of speakers) and that ever present and hugely annoying "an error occurred" message will be gone from your life forever.

-)

Are you running the computer as an administrator?
Do you own your internet connection or are you connection, via WiFi to a network?

Some networks/computers have a filter that can disable YouTube videos.
This mostly occurs on school computers.
If you are not the computer/network administrator, please contact them and report your problem.

Yes Sir You Da Man bobsleftnut ! I Downloaded the YouTubeCenter add-on for Firefox and there I go !!! F*ck Dash !!!

Directly is there : https://addons.mozilla.org/en-us/firefox/addon/youtube-center/

Disable Dash and Disable SPF too, it helped me.