Had the same problem, checked and implemented the solutions and none of them worked.

Reset Firefox and back to normal. (Help. Troubleshooting information)

Did note that I have two versions of Shockwave flash 11.7 and 11.8, is this correct or there be only one?

Hi Momoe, you should only have one Shockwave Flash plugin at a time. Sometimes if the plugin is in use, the Adobe updater will leave the old one. In that case, you might need to delete it manually. The location depends on your operating system and version. For now, perhaps easiest to disable 11.7.

I have had this problem with Win XP over many revisions of Flash/FireFox and tried many of the (reasonable) suggestions including ALL of those from the helpful jscher2000. Currently Flash 11 (as usual) displays a message in FireFox (23 at present) that it has installed successfully. But there is no Flash plug-in (enabled or disabled) in the FireFox plug-in list. Given the very long history of this problem reported by many users, it seems incredible there has not yet been a definitive solution.

Hi sayang, can you confirm:

• The plugin is in the customary location which I think for XP is:
  
  C:\Windows\System32\Macromed\Flash

• If you are comfortable using RegEdit, the Adobe Flash plugin is registered in this key:
  
  HKEY_LOCAL_MACHINE\SOFTWARE\MozillaPlugins\@adobe.com/FlashPlayer

• You have Firefox's plugin scan enabled (as described on the first page of this thread in this post: https://support.mozilla.org/questions/960667?page=1#answer-442297

I discovered the real cause of the (my) problem:

The data for the registry key

HKEY_LOCAL_MACHINE\SOFTWARE\@adobe.com\MozillaPlugins/FlashPlayer referenced plug-in 11.7.700.224

However the dll in folder

\Windows\System32\Macromed\Flash was revision 11.8.800.94.

I don't know how many other registry keys reference the 11.7.700.224 version, so as an expedient quick fix I copied as follows:

copy NPSWF32_11_8_800_94.dll NPSWF32_11_7_700_224.dll.

Restarting Firefox and looking at the plug-in list, at last there was: Shockwave Flash 11.8.800.94

I tried a video that previously failed and it worked!

I remain incredulous as to how the registry entry/entries was/were not updated with the installation of the new flash player.

Sorry jscher2000!

I did not notice your recent suggestion about the registry key prior to discovering it myself. If I had seen your suggestion earlier, it would have certainly lead directly to a solution.

Hi sayang, no worries. I've never seen that one before. Perhaps the installer got hung up on a prior task and never updated the old entry, or maybe Windows locked it for some reason. You've shed light on why some people fix Flash problems by re-running the installer, which otherwise would be completely inexplicable as a solution.

CORRECTION:

An earlier post by me gave a key as:

HKEY_LOCAL_MACHINE\SOFTWARE@adobe.com\MozillaPlugins/FlashPlayer

It should be:

HKEY_LOCAL_MACHINE\SOFTWARE\MozillaPlugins\@adobe.com/FlashPlayer

To jscher:

In fact I ran the Flash installer many times, downloading it `fresh' upon each occasion.

I have checked on another PC (upon which I rarely play videos) and found that the FireFox plugin list included Shockwave Flash version 11.8.800.94 and the corresponding registry value was also 11.8.800.94. It could be that on the `problem PC' there is a problem with registry permissions (if there is such a concept).

Good job, I fixed mine by doing a reset on Firefox, wish I'd seen this first!

Problem solved! I downloaded and installed Firefox 24.0 Beta. Apparently Mozilla programming team made all the right changes in the new release. Now all videos starting working on Yahoo and BBC.

Link: http://www.mozilla.org/en-US/firefox/channel/#beta

(Bad "Solution" removed)

Downgrading to an old version of Firefox is never a good idea, so please don't follow the suggestion above (which I removed)