User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:83.0) Gecko/20100101 Firefox/83.0

Steps to reproduce:

Install the firefox flatpak package with its dependencies on a Ubuntu 20.04 system.

$ flatpak install org.mozilla.firefox
ID Zweig Op Remote Download

1. [✓] org.freedesktop.Platform.GL.default 19.08 i flathub 88,6 MB / 89,1 MB
2. [✓] org.freedesktop.Platform.Locale 19.08 i flathub 23,5 MB / 318,3 MB
3. [✓] org.freedesktop.Platform.VAAPI.Intel 19.08 i flathub 8,6 MB / 8,7 MB
4. [✓] org.freedesktop.Platform.openh264 2.0 i flathub 1,5 MB / 1,5 MB
5. [✓] org.gtk.Gtk3theme.Yaru 3.22 i flathub 171,7 kB / 228,5 kB
6. [✓] org.freedesktop.Platform 19.08 i flathub 193,3 MB / 238,5 MB
7. [✓] org.mozilla.firefox.Locale stable i flathub 493,0 kB / 44,7 MB
8. [✓] org.mozilla.firefox stable i flathub 78,9 MB / 79,9 MB

Then try the site https://www.tagesschau.de/multimedia/sendung/ts-40109.html .

Actual results:

The video playback is very slow and stuttering. Installing the package org.freedesktop.Platform.ffmpeg-full solves this issue and results in a fluent video playback.

Expected results:

The video playback is fluent by default.