Security Issue: Update your Flash Player Version 19.0.0.245
https://www.adobe.com/products/flashplayer/distribution3.html Note: Windows users should download the ‘’’active_x’’’ for “Internet Explorer,” and the ‘’’plugin’’’ for “Plugin-based browsers” (like Firefox).

Note: Edge on Win10 does not use ActiveX version but its own Flash.

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

Separate Issue; Your System Details shows;

Installed Plug-ins

Adobe PDF Plug-In For Firefox and Netscape 10.1.9 Adobe PDF Plug-In For Firefox and Netscape 10.1.16
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Adobe PDF Plug-In For Firefox and Netscape: Adobe Reader Version 2015.009.20069 Note: McAfee Security is offered in the download.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

In past threads, this problem has been linked to malware. Our support article lists free cleaning programs other Firefox users have found helpful: Troubleshoot Firefox issues caused by malware.

Your MindSpark Toolbar is a well-known trouble maker. Google for information and removal advice.