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.
ప్రత్యుత్తరాలన్నీ (2)
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.