Mali is only for rendering, video decoding is done on separate HW core. However, its driver is not finished, VAAPI lib for it is also not finished yet and most of all, firefox supports HW video decoding (e.g. VAAPI) only on wayland. So, nothing that you could use right now. If you want decent video player on OPi Plus2E you can use LibreELEC image (Kodi) which has all work in progress included, but that is probably not something you had in mind (multimedia center app with no browser but youtube addon can be installed).