Please check if this happens in a new profile. If the new profile is okay, you can later copy the needed personal data from the old profile. Firefox stores your personal data and settings in another location (profile folder) separate from its files/folder. A new profile would have the default Firefox settings (Tools (Alt + T) > Options, and about:config ), and usually would also be empty of any Extensions and themes (Appearance) in Tools > Add-ons) and their settings. Also, a new profile would have no previous stored website data/settings etc., (Tools > Clear Recent History).
Thanks Vivek. Unfortunately, the problem persists with a new profile. Hopefully the next update to Firefox 11 may resolve the issue.
A possible cause is security software (firewall) 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.
Did you check the content of the hosts file to see which sites were blocked?
That is an IP in Hungary.
Any suggestions as to how it got into my Hosts file?
Possibly done by malware.
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.
You can also do a check for a rootkit infection with TDSSKiller.