If it isn’t a site that has an open tab or you are actually connecting to it ?
Then it is likely to be a new/recent/updated browser add-on, does this ring a bell ?
Adding an exception to a site if my questions above wasn’t you trying to connect to the site isn’t wise.
The fact that your exception is incorrect as the alert is for https not http that is in the image you posted. That said it appears that the site doesn’t support the https protocol, so firefox it most likely trying to connect to the secure https protocol, still resulting in an alert.
But before modifying that exception, I would suggest investigation is the course of action of less risk.