You need the x264 media player in Firefox by Cisco which is usually already built-in (do not install anything). Please check it by looking at the list of installed plugins within Firefox: Menu: Add-Ons -> Plugins
Reference: Firefox's Open-H.264 plugin
You could, now, however, be hit by this new bug here:
Bug: Firefox for Android no longer automatically downloads the Cisco OpenH264 plugin as of version 68
Since this change was introduced by Mozilla in version 68, you could downgrading to a lower version - perhaps the latest LTS version?! - and try playing the video. According to Mozilla (second reference above), an existing plugin will not be uninstalled which means that once you have the plugin (check the Firefox menu), you could try upgrading again.
If you have installed the mentioned plugin and still cannot play some HTML5 content, here is an entry in openSUSE Linux's support database:
SDB: Firefox MP4/H.264 Video Support
(in short, due to patent issues some packages are missing from openSUSE but the well-known Packman repository for SuSE provides the necessary libav*
packages. They are also automatically pulled in if you enable the Packman repo and then install vlc
from that repo (select it in yast2 sw_single
) or use zypper
).