Occasional Clipboard Failure

• Rich text editors, such as Excite and Hotmail, sometimes block access to the clipboard when used with Firefox. If you wish to use the clipboard with some rich text editors, you may need to use a different browser, like Internet Explorer or Chrome.
  
  Microsoft Remote Desktop is known to cause occasional problems with the clipboard. Disable Microsoft Remote Desktop to restore clipboard access. To disable it, click "Start." Right-click "Computer" and click "Properties." Click "Remote Settings" and "Don't allow access to this computer."
  
  If you open several windows at once with a quick launch or desktop shortcut, you may have problems using the clipboard. Minimize and maximize all windows to restore access.
  
  

Constant Clipboard Failure

• Malware or other malicious software that infects and takes over your computer sometimes disables clipboard access. Pushow, for example, is one malware that Mozilla identifies as problematic. To eliminate malware, run an anti-virus program, such as McAfee, AVG or Norton. Run the program regularly to check for new infections.
  
  A specific registry string is known to cause problems between Firefox and the clipboard. This string is sometimes related to malware, or it may point to a legitimate application. If you feel comfortable editing the registry, try deleting this string. Close all applications, and then click "Start." Type "regedit" in the search bar. Press "Enter." Cick the registry entry "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows." A string named "Applnit_DLLs" appears in the right pane. If you see a number next to the string, it may indicate a malware infection. Right-click "Applnit_DLLs" and click "Delete."
  
  

Extension and Plugin Problems

• Extensions and plugins are created by the Mozilla Firefox community, and they occasionally cause a conflict with the clipboard. Disable your plugins and extensions temporarily and try to use the clipboard again. An extension called AllowClipboard Helper lets you use the clipboard on websites that normally block its use. Download it for free at the Mozilla website.
  
  If you prefer not to download the AllowClipboard Helper extension, making a security policy for Firefox may restore clipboard access on some websites. In the address bar, type "user.js." Press "Enter." Add the following lines: user_pref("capability.policy.policynames", "allowclipboard");
  user_pref("capability.policy.allowclipboard.sites", "http://www.example.com");
  user_pref("capability.policy.allowclipboard.Clipboard.cutcopy", "allAccess");
  user_pref("capability.policy.allowclipboard.Clipboard.paste", "allAccess");
  
  Replace "http://www.example.com" with the websites that cannot use the clipboard. Save the changes and restart Firefox.
  
  

Reinstall Firefox

• Restarting your computer is one way to fix some problems with the clipboard. If, after you restart your computer and re-open Firefox, you still cannot use the clipboard, you may need to reinstall Firefox. Download a new version of Firefox from the official Mozilla website and save it to your desktop. Close Firefox. Click "Start," "Computer" and "Local Disk (C:)." Open "Program Files." Delete the "Mozilla Firefox" folder. Reinstall Firefox by double-clicking on the installer on the desktop.