Do the other alternatives work with Insert and Delete?

Use keyboard shortcuts if the buttons on the web page or other methods aren't working.

• Copy: Ctrl+C or Ctrl+Insert (Mac: Command + C)
• Paste: Ctrl+V or Shift+Insert (Mac: Command + V)
• Cut: Ctrl+X or Shift+Delete (Mac: Command + X)

Is this with one website, several, many?

https://support.mozilla.org/en-US/kb/update-firefox-latest-version 44.0.2

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

The problem persists in safe mode as well. And yes, newest version. Also the problem occurs on several sites, not only one.

Edit: But I just noticed that the problem also exists in other programs, not only Firefox... So sorry, then then this does not appear to be a Firefox issue.

Start your Computer in safe mode with networking. Then start Firefox. Try Safe web sites. Is the problem still there?

Starting The Computer In Safe Mode;
Free Online Encyclopedia

Do a malware check with several malware scanning programs on the Windows computer.

• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked