I installed openSUSE Leap 42.3 and I can't play online videos using Firefox. Although YouTube videos work fine Twitch videos don't.
What is the problem? How do I fix it?
Generally, you will need to a install community-hosted codec repository. A well-known repository is opensuse-community.org, with one-click install.
You can view which codecs are available to your browser (on YouTube) at youtube.com/html5. Twitch may use different codecs. Check their troubleshooting page at help.twitch.tv.