No, Firefox does not have a feature to permanently allow mixed active content on particular sites. I think the reason that this was not added is that there might unexpectedly be different mixed content in the site's various pages, so you might want to investigate each individual case.

Are there particular sites where you notice this problem? It shouldn't be that common. Do you use HTTPS Everywhere or another add-ons that forces HTTPS? If a site isn't really designed for HTTPS and you force it, then mixed content errors are much more likely.

Thank you for your answer. I'm talking about one domain only - my work place, university site. I do not use such add-ons.

Hmm, what does your IT suggest doing about this? Perhaps they suggest using a different browser, but all browsers now are stricter about mixed active content so that's not really an answer.

I always prefer Firefox. This is is the first time I can see a really big problem here. How about just create such a whitelist in the browser, even as add-on?

Perhaps an add-on could automate the giving of page-by-page approval on a site. I don't know enough to predict whether it would work.

You can suggest new features on the following page (click the sad face):

https://input.mozilla.org/feedback/firefox

Thank you, I'll try it.