I’ve seen something similar running Firefox on Mir. It could be that you need to add user_pref("gfx.webrender.all", true); to user.js. The following script handles this for Firefox installed as a .deb. If you’re using Firefox from a snap, you may have to alter it to update a profile under ~/snap/firefox:
https://discourse.ubuntu.com/t/using-firefox-wayland-on-mir-servers/19704