Please have a read here http://kb.mozillazine.org/Error_loading_websites Could be a IPv6 error, or not as it could be the sites issue. Please : Clear the cache and remove cookies only from websites that cause problems.

"Clear the Cache":

   Firefox > Preferences > Advanced > Network > Cached Web Content: "Clear Now" 

"Remove Cookies" from sites causing problems:

   Firefox > Preferences > Privacy > "Use custom settings for history" > Cookies: "Show Cookies" 

Then try this : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

Then: https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Please let us know if this solved your issue or if need further assistance.

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