It sounds like you are using the plug-in in a private window without approving the plug-in to work in private windows. Firefox recently started requiring that each plug-in be authorized to work in private windows. Go to your Add-Ons page in Firefox, click the 3 dots on the right of the add-on name, select More Options from the menu that appears, then click Allow next to "Run in Private Windows".

You must have had an older version of the extension. It was updated to deal with the shutdown of Norton ConnectSafe, which was previously used as the backend filter. We now use OpenDNS porn+malware filtering on the backend. As long as you had an updated extension, there should have been no lag time where filtering was broken.

We are at the mercy of whatever DNS filtering service we use on the backend, which is often not as configurable as we'd like. As of July 2019, the service we use is OpenDNS porn+malware blocking DNS.