It's been amazing. With the ad-blocker she almost never texts me now with suspicious stuff. Also, it was super easy to teach her that if she ever gets into a situation that feels shady, just close the browser and open it up again. Starting from a clean slate every time in a well protected browser makes her feel a lot safer because she knows that if things get scary, she can just close the window and start over.
Having to log in every time hasn't turned out to be that much of an inconvenience either. See actually feels safer because of it. It makes perfect logical sense to her that the website ask her to login every time. "My bank asks me for my ID every time I go in to deposit a check, so of course it makes sense to ask for my password every time I open my browser and visit the website."
Anyway, my takeaway is that Firefox + uBlock Origin for parents is really a wonderful thing.