Hi it may be possible, but this will depend on the site that you are downloading from and any content protection that has been put in place. This [https://support.mozilla.org/en-US/kb/find-and-install-add-ons-firefox-android guide] will walk you through how to install add-ons for Firefox for Android and this [https://addons.mozilla.org/en-GB/android/collections/mozilla/watching-videos/ list] has some that you may find useful.