Which extensions do you have installed?

Are you using a proxy?

• Tools > Options > Advanced : Network : Connection > Settings

If it works in Firefox Safe-mode then disable all extensions (Tools > Add-ons > Extensions) and then try to find which is causing it by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

cor-el,

    I only have one extension installed, and that was recommended to me by a contributor here, capture and print 1.6   So, obviously if that is the only extension then that must be the culprit? Any suggestions as to why that is a problem cor-el, if in fact that is the problem? I purposely installed it as per a recommendation from this forum so I could print what my computer monitor screen shows. If that extension is the problem, then how do I try and keep the feature of being able to print from what I'm viewing on my monitor? Thank you, cor-el.

Did you try to disable that extension to verify that it is causing the problem and that not something else is causing it?

In Firefox Safe mode these changes are effective:

• all extensions are disabled
• the default theme is used (no persona)
• userChrome.css and userContent.css are ignored
• the default toolbar layout is used (localstore-safe.rdf)
• the Javascript JIT compiler is disabled
• hardware acceleration is disabled

• plugins are not affected
• preferences are not affected

cor-el,

    I will disable that extension and see if the problem goes away. How do I make those changes you suggest in safe mode?

You can check and change the status of an extensions on the Tools > Add-ons > Extensions page.

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

cor-el,

      Maybe we're getting ahead of our self's here. I thought your suggestion about safe mode was in reference to  changes that can be made in safe mode. But, maybe that wasn't want to you meant? Can you clarify your statement about safe mode cor-el, in which you state:   

In Firefox Safe mode these changes are effective:

   all extensions are disabled
   the default theme is used (no persona)
   userChrome.css and userContent.css are ignored
   the default toolbar layout is used (localstore-safe.rdf)
   the Javascript JIT compiler is disabled
   hardware acceleration is disabled 

   plugins are not affected
   preferences are not affected 
Was that just a statement about what happens in safe mode, or can these  changes be made while in safe mode? Please clarify. Thank you.

cor-el,

   Ran the system for awhile, while print screen was deleted. At first I thought the problem was gone. But, the problem has returned. So, deleting the print screen extension function had no affect on solving the problem. Being the only extension was print screen capture, and that was not the problem, what is the next option to try and solve the issue?

Which security software (firewall, anti-virus) do you have?

A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls

Do a malware check with some malware scanning programs on the Windows computer.
You need to scan with all programs because each program detects different malware.

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
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender: Home Page:
  http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
• 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.

• http://support.kaspersky.com/viruses/solutions?qid=208280684

See also:

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

I'm having the same problems. I think it's a bad modem. Look to see if the online light on your modem is off or flashing when you're having these problems...it should be a steady green.