Try to create a new profile as a test to check if your current profile is causing the problems.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Did you post this on the fast or slow PC? Check that your plugins are up to date. Also, some users had similar problems and found they had multiple plugins of the same name but different versions.

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Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Did this fix your problems? Please report back to us!

Thanks for the suggestions but none of the following helped:

a) Start in Safe Mode

b) Creating a new profile

c) Running (for many hours)

   Microsoft Safety Scanner
   MalwareBytes' Anti-Malware
   TDSSKiller - AntiRootkit Utility
   Hitman Pro
   ESET Online Scanner

d) Uninstalling, deleting folder "Mozilla Firefox", re-installing

Running a standard internet speed test on the `bad' PC produces:

IE8 about 730 KB/sec

FF 27.0.1 about 88 KB/s

On the `good' PC, the speeds are much the same at about 730 KB/sec.

Looking at the network data, there does seem to be a problem with an Alert (Encrypted Alert (21) ). If such occurs, it seems to take over 10 seconds to recover.

Four network data dumps (*.txt) can be found at:

www.discway.com.au/cmad

a) GIS1 and IE8 (Bad PC)

b) GIS1 and FF (Bad PC) # Aborted prematurely as test not finished within 6 minutes.

c) GIS2 and IE8 (Good PC)

d) GIS2 and FF (Good PC)

Have you tried disabling Google Safebrowsing (turn it back on when finished)?

• How does built-in Phishing and Malware Protection work?

To disable it, go to Firefox > Options > Security Tab > Uncheck Block reported attack sites and Block reported web forgeries.

A restart of firefox may be necessary for it to take effect.

Start Firefox in Safe Mode While you are in safe mode;
Firefox Options > Advanced > General. Look for and turn off Use Hardware Acceleration.

If there is still a problem, Firefox > Add-ons > Plugins. Disable EVERYTHING, re-start and try again.

Thanks for the recent suggestions Waka... and Fred.. Plugins, Extensions, Safebrowsing,, Block attack site, Block forgeries were all disabled during the various investigation phases without result.

NEW INFORMATION:

Resetting Firefox (Help | Troubleshooting | Reset) appears to (at least partially) clear the problem as long as one does not resize the default reset window and as long as one does not restart Firefox. This would seem to indicate malware targeting Firefox or an incredibly abstruse program bug. If it is malware, it appears to infect the memory image given a change (eg resizing the window) and is saved upon exit, but can be cleared by another reset.

The malware tests run some time ago did not reveal anything untoward.