So my problem seems to be different that the others I've seen like this. I have my own HTML files saved on my phone that I was able to open in Firefox before with them rendering correctly and all, but now when I try to open them, they just show the code in text form. Here's what I tried: Refreshing, disabling AddOns, clearing cookies, cache, you name it; uninstalling and reinstalling Firefox, and rebooting the phone. The other things to bear in mind is that my file is a genuine HTML file. It doesn't have any crazy rich text markup or have a .txt extension or anything. The file also passes the W3C Markup Validation with no errors and no warnings. Like I said, the files rendered before just fine. There is this HTML viewer that Android comes with that still renders these files. I am also not rooted or anything either. I'm running Android 9.0 "Pie" with Firefox 68.11.0. I believe this is my fault, in that I must've touched something I shouldn't have, but alas. Is there something that could be right under my nose that I'm missing? Or maybe a value in about:config I have to put in? (My about:config is precisely as it is when it is downloaded fresh from the Play Store.) Can you help me?