It is possible that your security software (firewall, anti-virus) 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

• https://support.mozilla.org/kb/fix-problems-connecting-websites-after-updating

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

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

Check the connection settings.

• Tools > Options > Advanced > Network : Connection > Settings
• https://support.mozilla.org/kb/Options+window+-+Advanced+panel

If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/kb/Firefox+cannot+load+websites+but+other+programs+can

See also:

• "Hang at exit": http://kb.mozillazine.org/Firefox_hangs
• "Firefox hangs when you quit it": https://support.mozilla.org/kb/Firefox+hangs

You may also want to check for malware.

• 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.

It just happened again for the first time since my last post.

For Antivirus I use Notion Internet Security. Last week I set the firewall to allow Firefox automatically but that doesn't seem to have helped. I have done a couple of full scans since this problem first occured and it didn't find any malware.

At the time of the crash I had no other active programs running. Fiefox has also not updated since the last post. According to Task Station 68 other processes were running at the time. Who knows?

I am still at a loss to see what is causing this. It is difficult to analyse because only happens about once a week even though I use firefox every day.

Norton you mean? Check with all the tools i suggested, can you reproduce in safe mode with networking.

• http://windows.microsoft.com/en-us/windows/start-computer-safe-mode

As a test you can try to load some web pages via Google Translate with a random source language.

• http://translate.google.com/translate?hl=en&sl=fr&tl=en&u=<append URL>

This problem has not occured in the last few weeks and may have been solved in the Firefox updates 25.0 or 25.0.1.

It was previously happening intermittently about once a day. I was never able to reproduce the problem reliably, so the original cause remains unknown.