Subtitles display intermittently in Firefox. For example, try to play https://commons.wikimedia.org/wiki/File:%C2%BFQu%C3%A9_es_Wikipedia%3F.ogv from https://meta.wikimedia.org/wiki/Grants:IEG/Motivational_and_educational_video_to_introduce_Wikimedia/Scripts#The_principles_and_WP:FIVE_Pillars_of_Wikipedia.
If a file is displayed on Commons and a language is selected in the "Language select" box instead of or in addition to using the CC button, the subtitles sometimes display.
I have not determined the exact combination of uses of the "Language select" box and the CC button that cause subtitles to display properly. Regardless, the user should not need to struggle in order to get subtitles to display.
Also, it's confusing to have both a "Language select" box and a CC button which may be selected to conflicting languages.