Pressing ctrl will make firefox hang.
As the question states, simply pressing the ctrl key makes firefox hang. So doing things like ctrl + a, ctrl + s, and the like makes it hang/not respond. It's saying Warning: Unresponsive Script and then varies from there.
Some screenshots here since the uploader in the form won't work for me.
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Try the Firefox Safe Mode for troubleshooting this issue.
I would suspect a "Tab" extension that "isn't playing well with others" as the probable cause, based on those messages.
Separate Issue; Your System Details shows;
Installed Plug-ins
Adobe PDF Plug-In For Firefox and Netscape "9.5.5" OutOfDate
Adobe PDF Plug-In For Firefox and Netscape:
Adobe Reader Version 2015.009.20069
Note: McAfee Security is offered in the download.