That's bizarre. I am also on Windows 10 and use Firefox as my primary browser, largely because I can stream DRM'd video sites (Netflix etc) to my friends on discord.
Sounds dumb, but have you tried uninstalling and reinstalling? I might suggest also removing or disabling all extensions to see if that does anything.
Once Firefox on mobile got extension support, I switched over immediately to use a decent adblocker. Made sure every app that opens a browser opens in Firefox. Has made my mobile browsing experience so much better, of my goodness.