I should just preface this by saying: the 64-bit version of Firefox may have special issues that I know nothing about.

Could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement? (More info: Diagnose Firefox issues using Troubleshoot Mode)

Thanks for the reply jscher,

I'm running the 64bit version indeed. Safe-mode doesn't help either unfortunately. It's very strange - the problem persists even throughout backup restoration... Disabling my AV doesn't help. My Firewall settings haven't changed and Flash content does work in other browsers.

One thing I have been told to try is disable hardware acceleration of Flash itself, but I can't enter the Flash menu without Flash working in the first place I don't think.

One other common troubleshooting step is to disable the protected mode feature of the Flash player plugin. That feature has security benefits, but seems to have serious compatibility issues on some systems. You can disable it using the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. On the right side, find "Shockwave Flash" and click the More link. Then uncheck the box for "Enable Adobe Flash protected mode" and see whether it helps. You might need to exit Firefox and start it up again before it takes effect.

The option wasn't in the plugin menu, so I had to set "dom.ipc.plugins.flash.disable-protected-mode;true" - that's the same thing, right?

Didn't resolve the issue either though I'm afraid. I have also tried FF beta version 45 without success. Maybe I should try an old Flash version next.

Did you use the Flash uninstaller between installs? There's a link to it here: https://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html

You can try the "Extended Support Release" of Flash 18, which is still getting security updates. Adobe is threatening to discontinue this page, but it seems to be there this morning:

https://www.adobe.com/products/flashplayer/distribution3.html

Scroll about 2/3 of the way down the page, and in the table for that release, look for the row for "plugin-based browsers" and you can use either the EXE or the MSI installer.

To roll back from 20 to 18, you probably need to use the uninstaller I mentioned earlier...

Thanks for that, I was actually struggling to locate an older version.

I did use the uninstaller before - even deleted the remaining folders and restarted. The old version is displaying the same problem.

Seeing as how a reinstall of Firefox didn't help either, are there some registry leftovers or anything I should remove for a truly "clean" reinstall? After uninstalling I removed the Mozilla Firefox program folder and the Mozilla folders both in Roming and LocalLow. But maybe there's something else I have to do before the new install is truly virgin?

I doubt there are any registry settings that make a difference.

Is Flash nonfunctional even on Adobe's demo pages:

• https://www.adobe.com/software/flash/about/
• https://helpx.adobe.com/flash-player.html

In both cases, the final part of the version number can't display without Flash running.

Do you have any kind of filtering set up, for example, through your security software?

Nope, not working there either; it stays white and I get the loading symbol when I click on the areas. The version number is only displayed as "20.0.0" too.

Nothing special in my security software, and nothing that I would have changed coincidingly with the issues appearing. I can also disable real-time protection and Flash still doesn't run properly.

Luckily most sites support HTML5 these days, but there are some Flash-exclusive things I do still.

Is Flash set to "Always Activate" (default setting) or "Ask to Activate" (show show an icon on the address bar) on the Add-ons page:

• Ctrl+Shift+a or
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. Look for "Shockwave Flash" on the right side. All normal?

Same result for both "ask to activate" and "always active". The plugin page as well as about:plugins (and dxdiag) show no problems.

I also tried different "dom.ipc.plugins.sandbox-level.flash" settings (0, 1, 2, 3) with Firefox restarts inbetween, but no success.

Ok, I just installed a fresh copy of the newly shipped Firefox 45 and the problem has resolved itself; Flash works! Thanks for trying to help.

Still strange...

stirner said

Ok, I just installed a fresh copy of the newly shipped Firefox 45 and the problem has resolved itself; Flash works! Thanks for trying to help. Still strange...

The problem appeared in my Firefox 45. Clear reinstalling with revo uninstaller doesn't help. Plugin-container copies itself in memory many times and video does not work.

Hi LiXiQing, I'll reply to your newer question over here since this one already is marked as solved and your symptoms look very unique: https://support.mozilla.org/questions/1116085