firefox switches from big to smal in fullscreen at visiting videos from atv.at

[Thomas Eichinger]

watching the movie (https://www.atv.at/mein-recht-staffel-2/folge-2/d429784/) in full screen results in a switching between fullscreen and small image without user action.

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: firefox 71.0+build5-0ubuntu0.19.10.1
ProcVersionSignature: Ubuntu 5.3.0-24.26-generic 5.3.10
Uname: Linux 5.3.0-24-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu8.2
Architecture: amd64
BuildID: 20191205203726
CurrentDesktop: ubuntu:GNOME
Date: Wed Dec 11 00:07:29 2019
DefaultProfileExtensions: extensions.sqlite corrupt or missing
DefaultProfileIncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
DefaultProfileLocales: extensions.sqlite corrupt or missing
DefaultProfilePrefErrors: Unexpected character ',' before close parenthesis @ /usr/lib/firefox/omni.ja:greprefs.js:722
DefaultProfileThemes: extensions.sqlite corrupt or missing
InstallationDate: Installed on 2019-12-05 (5 days ago)
InstallationMedia: Ubuntu 19.10 "Eoan Ermine" - Release amd64 (20191017)
ProcEnviron:
 LANGUAGE=de_AT:de
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_AT.UTF-8
 SHELL=/bin/bash
Profiles:
 Profile1 (Default) - LastVersion=None/None (Out of date)
 Profile0 - LastVersion=71.0/20191205203726 (In use)
SourcePackage: firefox
UpgradeStatus: No upgrade log present (probably fresh install)