Susan I really can't help you with this because I've never made a secondary user account BUT I thought only the primary account was able to install/download add-ons and programs so I would assume that a secondary user couldn't add Adblock Plus to their browser. But if the Adblock Plus icon showed up on his old user account then maybe all you need to do is reboot the pc for it to show up or you could just go to a website that normally has ads such as cnn.com and see if you see any ads. If not then adblock plus is working, the icon just isn't there.
I also assume that Avast would be set up to protect your whole pc including secondary accounts ? did the avast icon show up in his old user account ?
As long as you are setting up a new user for your husband did you already delete the old user ? I'm asking because here's an article on how to get rid of MediaTab.TV on Firefox-
https://www.pcrisk.com/removal-guides/10138-search-mediatab-tv-redirect#a5