That message is characteristic of a Web Application Firewall that sites use to deflect bot and attack traffic. You could get that message if your IP address is part of a block of banned addresses, or if something about your request looks suspicious to the perimeter defenses.

Are you using a proxy server or VPN that changes the appearance of where you are connecting from?

Do you use any privacy-related add-ons that could be modifying the information Firefox normally sends to servers?

You also could try clearing the accumulated cookies for the problem sites in case they are causing the problem. Here's how:

When the error comes up, with the site's address in the address bar, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that.

Firefox will ask you to confirm removing cookies for the current site.

Any improvement?

You can remove all data stored in Firefox for 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") to see if that works.

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for 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 involved files.

If you revisit a 'forgotten' website then data for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

Looks like the issue was with my VPN - thank you for the timely help!