You should not be using Firefox 4 it is no longer supported or secure. Upgrade to Firefox 5 or 6 using an official link.

If the problem re-occurs please post a screenshot of the message you are getting. you may then need to go through a clean reinstall procedure, but do not use the option to delete settings, because that will normally remove history & passwords etc from any & all installed versions of firefox. (There is a specific preference or js file that may sometimes need deleting). See installing Firefox on Windows

This issue can be caused by the presence of leftover files in the Firefox program folder (defaults\pref) like a file firefox.js that overrides the update URL with a wrong link.

Do a clean reinstall and be sure to remove the Firefox program folder to remove the file(s) that cause the problem.

See:

• [/questions/826858]

My father has this very same problem with Seamonkey on Windows XP, so it seems it's a general Mozilla problem.

My father's machine has been in use for years and regularly updated everything installed (Windows, Avira Antivirus, MSOffice, Adobe Reader, Seamonkey) without any problem. There's no sign of a compromise or malware infestation.

The message appeared out of the blue and it's quite unhelpful, particularly the part on contacting the network provider. No ISP I asked knows anything about that message, how it comes about and what to do about it, and I can't blame them for that. So at the very least it should be reworded.

For the time being, the best course of action seems to be to ignore the message and try not to be too annoyed by it. (It *is* rather spectacular and alarming.)

Did you check for leftover files in the defaults\pref folder in Firefox program folder?

There should only be a channel-prefs.js file in the defaults\pref folder.
All other files and folders that used to be in the defaults folder have now been moved to a ZIP archive called omni.jar to speed up the Firefox start and any leftover files and folders should be removed.