I added a custom block page following steps I found here and in some other pages that did the same steps. When I try to access a blocked page through Mozilla, everything works fine, giving me this result:
Not without making a certificate that will work with https sites. This would involve setting up a MITM on your Pi with a trusted cert, etc. Not recommended.
The block page option will likely be removed in a future Pi-hole version due to it's very limited usefulness. Only a very few sites on the internet use http.