Let's start with this;

Your System Details shows;

Installed Plug-ins

Shockwave Flash 26.0 r0 Shockwave Flash 16.0 r0
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Flash: Grab the uninstaller from here:

http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html Uninstall Flash Player | Windows

http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-mac-os.html Uninstall Flash Player | Mac

Then reinstall the latest version.

Note: Windows users should download the active_x for Internet Explorer. and the plugin for Plugin-based browsers (like Firefox).

Note: Windows 8 and Windows 10 have built-in flash players and Adobe will cause a conflict. Install the plugin only. Not the active_x.

Flash Player Version 26.0.0.151

https://get.adobe.com/flashplayer/ Direct link scans current system and browser Note: Other software is offered in the download. <Windows Only>

https://get.adobe.com/flashplayer/otherversions/ Step 1: Select Operating System Step 2: Select A Version (Firefox, Win IE . . . .) Note: Other software is offered in the download. <Windows Only>

See if there are updates for your graphics drivers https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

FredMcD- thank you for your detailed assistance! You were right, I had 2 versions installed so I removed the old one by following your instructions. Now I have only the latest 26 version but still I am experiencing the same issue! :(

What I did: 1. Ran uninstaller and restarted FF 2. Ran latest version installer from the adobe site 3. There was still both v16 and v26 active - turning off one turns off the other automatically, so I checked which file v16 runs from and removed it from my HDD (NPSWF32.dll) so I ended up with only v26 4. To be sure, I ran the uninstaller again and did a fresh install of the latest version 5. Still there is no change

I have attached a screenshot of my current plugins if that is of any help

PS: I tried opening the "problematic" site in a separate window (instead of tab) and did not minimize it - it's okay. The problem appears when you minimize or switch tabs - the window becomes completely un-interactive (even no right-click menu)

Well done.

How long is it until the mouse stops working?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

The mouse stops working as soon as I switch to the tab that was in the background... it must be something in the way Flash works with the latest versions' scripts... I opened FF in Safe Mode but the problem still persists - this time the plugin stopped responding and I got the error in the attached screenshot

If you have any further ideas, I'd be grateful to hear them - but in any case, you have been very helpful for which I thank you! I guess we'll just have to wait for the next release(s) and hope it will fix this glitch

That script is sent by the web site.

Location of the cache files; Windows: *C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile>\ Mac: ~/Library/Caches/Firefox/Profiles/ Linux: ~/.cache/mozilla/firefox/

Close Firefox. Open your file browser to the above, and remove the folder.

I understand... could you please specify which folder I should delete in the path provided above? (sshot attached)

The cache2 folder.

Thank you. I closed FF then deleted this folder and opened it again. Still the same :/

https://support.mozilla.org/en-US/kb/update-firefox-latest-version?cache=no Have you updated Firefox to the latest version 55.0.3

Make sure your system programs are up to date.

Start your Computer in safe mode. Then start Firefox. Try Safe websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

I updated FF to 55.0.3 from the start. I followed your instructions and I am currently still in Safe mode as I am writing this. Tried a bunch of other websites (I could not find a Flash-driven one) and tried the problematic one as well - no change

V55 has been giving users problems. Reinstall v54.

https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements

@FredMcD - Tried downgrading and v54 works like a charm (again) :)

Thank you very much for your patient and meticulous assistance! You have made my day a bit better. Take care!