Hello okehazaman, check it in [https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode#os=win7&browser=fx16 Safe Mode], in Safe Mode window select '''''"Start in Safe Mode"''''' and see if this happen again, '''''if not''''' see: [https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems#os=win7&browser=fx16 Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems] thank you Please mark "Solved" the answer that really solve the problem, to help others with a similar problem.

See also: *http://kb.mozillazine.org/Clipboard_not_working

I fixed the problem　by your advice. Thank you!!!