The Flash plugin may be disabled on your browser. To enable it again, please do so from the plugins manager.

For more information on enabling/disabling plugins, please refer to the Managing plugins section of this article:

• Use plugins to play audio, video, games and more

It is already enabled ("Always enabled" in the add-ons manager), but the issue still exists…

Please refer to this Mozilla support article for more information on fixing issues with video/audio playback:

• Fix common audio and video issues

Also, is the issue only with the SpeedTest website or does it occur on all flash content? Please visit this Adobe help page and let us know if you can see the flash logo in the article.

I can see the Flash logo on the Adobe website and also played the two videos (the one in HTML5 in Youtube and the other one in Flash showing Nelly Furtado singing)

(if it helps, on Linux Mint 17.2 it works)

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. Click the menu button , choose History, and then "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 Delete browsing, search and download history on Firefox article.

Did this fix your problems? Please report back to us!

No, it didn't fix… may it be a site-related issue?

It works correctly in my Firefox, so it is unlikely that it is an issue with the website itself.

Perhaps there is an addon interfering with the website. Can please try running Firefox in safe mode. Safe mode is used to diagnose issues related to addons because it temporarily disables all addons. Please try safe mode and let us know if the issue continues.

For more information on using Firefox safe mode, please refer to these Mozilla support articles:

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

I entered Safe mode and speedtest.net says again it requires Flash (and Javascript) enabled, but this should be normal since Safe mode disables all add-ons, am I wrong? What should I do now?

Plugins are not affected by Firefox Safe Mode, only extensions are disabled in Safe Mode.

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History") or via the about:permissions page.

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of the involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.

If the solution provided by cor-el doesn't solve the problem can you please try deleting the plugins.dat file from your Firefox profile? This file contains a database of your installed plugins. Deleting this file should force Firefox to rebuild this information.

For more information on locating your Firefox profile, please refer to these Mozilla support articles:

• Profiles - Where Firefox stores your bookmarks, passwords and other user data
• Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

Is "pluginreg.dat" the same? Because I can't find a "plugins.dat" file in my profile folder (unless I should navigate in the sub-folders)

Yes. The file that stores the plugins registry is the pluginreg.dat file in the Firefox Profile Folder.

See Re-initializing the plugins database:

• https://support.mozilla.org/kb/troubleshoot-issues-with-plugins-fix-problems

Thank you all :) Now it works, even if I need to refresh 2-3 times (but this happens also with other browsers, e.g. refreshing once is needed for Google Chrome)

I am glad to hear that your problem has been resolved.

Thank you for contacting Mozilla Support. Please feel free to get in touch again if you have more issues with Firefox.