Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache.

Note: This will temporarily log you out of all sites you're logged in to. To clear cache and cookies do the following:

• Go to Firefox > History > Clear recent history or (if no Firefox button is shown) go to Tools > Clear recent history.
• Under "Time range to clear", select "Everything".
• Now, click the arrow next to Details to toggle the Details list active.
• From the details list, check Cache and Cookies and uncheck everything else.
• Now click the Clear now button.

Further information can be found in the Clear your cache, history and other personal information in Firefox article.

As I said, I did everything suggested by Firefox in the trouble shooting section about this issue, including the above. Also disabled recent add ons. Nothing worked.

Did you try to remove all rules for Firefox from the firewall?

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

I had no firewall running. Just in case, I turned the Firewall on and added Firefox to the list of apps that would not be blocked, but still no change. As I mentioned before, I tried everything in the first link above. And I can reach all other websites, so the third link is not applicable.

You can try to disable IPv6 in Firefox.

See:

• http://kb.mozillazine.org/Error_loading_websites

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History") or via the about:permissions page.

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious and if you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of the involved files.

It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.

Try disabling the HTTPS-Everywhere extension, or make an exception for this site if possible. See: You block a computer website that Chrome and IE don't.

To review, configure, disable and remove extensions, try the Add-ons page. Either:

• Mac: Command+Shift+a; Windows: Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions.

FYI, this extension also is associated with crashes for some users.