checked https://www.mozilla.org/en-US/plugincheck/ to confirm plugin is updated. Updated other (unrelated) plugins and further testing still encounters this issue.
Source file is approximately 1 hour long. Confirmed uploaded file is the correct filesize. Tested in Safari and the embedded file plays fully.
Firefox is limiting the file to approximately 1 minute
Do you still have this problem ?
(If you solved it a link to any solution may help others)