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

I think I tried what you are saying. That is extremely difficult. I let Firefox have full permission. However, problem is not solved. If I simply use IE. I get the speed. When I use firefox it is rediculously slow loading websites. If I turn off Microsoft Security, then firefox works fine. But I don't want to do without my security. Looks to me like the Mozilla people have a problem that needs attention, because I am forced to go back to IE so I can get things done?

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

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

Do you have any network.http prefs that show as bold (user set)?

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold (user set).
• Preferences can be reset to the default via the right-click context menu if they are user set
• Preferences can be changed via the right-click context menu: Modify (String or Integer) or Toggle (Boolean)