Generally, you need to close Firefox to install the Adobe Flash. You can do so by clicking the 'x' button on top-right corner of the screen.

See: https://support.mozilla.org/en-US/questions/876769#answer-248034

If you are having any other issue, please do share a screenshot of the issue.

Thanks!

File > Quit/Exit is the proper way to close Firefox as the X on title bar only closes that one window.

Your System Details shows;

Installed Plug-ins

Shockwave Flash 22.0 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: Uninstall Flash Player | Windows 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: 22.0.0.192 Flash Player (Linux) Version 11.2.202.626

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

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.

FredMcD said

Flash Player Version: 22.0.0.192 Flash Player (Linux) Version 11.2.202.626

Those are the previous versions.

22.0.0.209 (Win & Mac) and 11.2.202.632 (Linux) have been the current since July 12.

Also for Windows users especially the https://www.adobe.com/products/flashplayer/distribution3.html (will be up till Fall sometime?) is the better place to download Flash Player from as Windows users can get bundled stuff if not careful on get.adobe.com/flashplayer/