Try this solution.
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- Disable or remove the addon 'Video DownloadHelper'. (Tools > Addons > Click Disable button or Remove button for Video DownloadHelper)
- Restart Firefox.
I have been trying to search for acceptable solution for many days because I had this problem. This solution worked for me. I posted the question as well as solution hoping it helps someone. Addons too can create problems.
qquito and some others, who had the same problem, were provided a workaround solutions such as downgrading the flash player version. There is no need of downgrading Flash Player. There seems to be a conflict between new version of Flash Player and Video DownloadHelper.
Such issues can also be caused by expired files in the cache.
Reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.
- Press and hold Shift and left-click the Reload button.
- Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
- Press "Command + Shift + R" (MAC)
Clear the cache and the cookies from sites that cause problems.
"Clear the Cache":
- Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"
"Remove Cookies" from sites causing problems:
- Tools > Options > Privacy > Cookies: "Show Cookies"
@cor-el I always clear the cache and and remove cookies, but it did not work for me. Reloading the page too didn't work. The problem reoccurred. Disabling Video Downloadhelper helped in my case.
I have this problem too but I find no such thing to disable. I checked in every category. There is no Video download helper.
Realplayer installed? Try to disable realplayer It will help!
I tried everything listed and more. Firefox simply will not play videos on youtube or on news websites. Looks like I have to dump firefox until they figure out what the problem is. Youtube plays in all my other browsers fine.
Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).
- Do NOT click the Reset button on the Safe mode start window or otherwise make changes.
You can check for problems with current Flash plugin versions and try these:
- disable protected mode in Flash 11.3 and later
- disable hardware acceleration in the Flash plugin
i tried to figure out various options (1) removing addons (2) updating flash players etc. finally i came to know that google chrome that i installed recently was the cause of this error in firefox. I uninstalled google chrome, and now youtube videos working well in my firefox. please comment if this solution has worked for you.