Thunderbird is a POP3/IMAP email that retrieves from the email server they must provide the protocols and logins for one to set Thunderbird to work with their email provider. Have you tried Private Mode browsing to see if the problem is same or work but make sure the Addons in Private Mode is disable when doing this test.

The "Access Denied" message with a Reference # is characteristic of a perimeter security system (also called a Web Application Firewall) that sites use to prevent bot and other suspicious connections to the website.

I notice that the URL you listed ends with a ? -- are there some parameters after the ? character like you might see on a search engine results URL? If not, one of your add-ons or privacy filters might be stripping them and making the URL invalid.

Otherwise, here are some possible issues to consider:

(A) If you are using a VPN when accessing these sites, try without the VPN.

When you are using a VPN, your IP address is from a remote system used by many strangers. So if that IP address is temporarily banned due to someone else's activity, that also will affect you. If you need the VPN, try selecting a different server or exit node in your VPN software.

(B) If you have customized any privacy preferences that affect browser requests -- for example, modifying your "user agent" or referring site header -- you may need to undo those changes

Also, some add-ons may modify headers in a way that looks suspicious to these systems. We can't really tell how your browser identified itself to the problem site based on how it identified itself to this site, because add-ons are restricted in how they interact with this site. Can you think of any add-ons you use that might make privacy-related tweaks to your requests?

(C) If you have Firefox set not to accept ANY cookies, that also could be a problem for some sites

(D) Maybe your requests are modified by an intermediary (such as a proxy server, your security software, or an interloper)

Although most people are not intentionally using a proxy server, Firefox may discover one set up in your system settings. Sometimes forcing Firefox to ignore those settings helps with weird connection issues. Here's how:

Open the Settings page using either:

• "3-bar" menu button > Settings
• (menu bar) Tools > Settings
• type or paste about:preferences into the address bar and press Enter/Return to load it

In the very tiny search box at the top of the page, type proxy and Firefox should filter to the "Network Settings" section of the page.

Click the Settings button, change the top setting to "No Proxy" and then click the OK button at the bottom of the dialog to save your change.

Any difference?