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

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Note that the latest Flash player for Linux is Flash Player 11.2.202.481 (7/10/2015)

You can find the latest Flash player versions for Firefox on this page:

• http://www.adobe.com/special/products/flashplayer/fp_distribution3.html

• https://www.adobe.com/support/security/index.html#flashplayer

Thanks for your reply!

The plugin check still doesn't work in Safe Mode. The default theme was used all along.

I updated flash manually, but the real issue here is the malfunction of the plugin check.

Hello,

The Refresh feature (called "Reset" in older Firefox versions) can fix many issues by restoring Firefox to its factory default state while saving your bookmarks, history, passwords, cookies, and other essential information.

Note: When you use this feature, you will lose any extensions, toolbar customizations, and some preferences. See the Refresh Firefox - reset add-ons and settings article for more information.

To Refresh Firefox:

1. Open the Troubleshooting Information page using one of these methods:
   • Click the menu button , click help and select Troubleshooting Information. A new tab containing your troubleshooting information should open.
   • If you're unable to access the Help menu, type about:support in your address bar to bring up the Troubleshooting Information page.
2. At the top right corner of the page, you should see a button that says "Refresh Firefox" ("Reset Firefox" in older Firefox versions). Click on it.
3. Firefox will close. After the refresh process is completed, Firefox will show a window with the information that is imported.
4. Click Finish and Firefox will reopen.

Did this fix the problem? Please report back to us!

Thank you.

I refreshed Firefox just a few weeks ago...

Firefox is my daily work tool, after Refreshing I lose several hours just by setting everything back to the state it was , because I heavily customize it to my needs. I'm not going to do that again unless there is no any other choice.

Since I just did it recently, and since we don't know if that will help at all, I'm very reluctant to do that and possibly give myself hours of extra work for nothing...

While Firefox settings are saved by Sync, several other important things are not, including plugin settings (like "classic theme restorer" - FF is useless without it), and website exceptions (like allow cookies).

As of today, the plugin check page started to work on its own!

Maybe it was a bug in the checker, maybe in something on my system that got fixed by the latest updates. I'll probably never know.