If your talking FB or YT then they have protections to prevent anyone to just right click and download the videos and requires 3rd party software outside of Firefox browser to do this for you. You can stream FB and YT videos but you are not permitted to just download them like any free streaming videos.

The websites in question are not fb or yt. They are subscription sites that provide videos for streaming as well as downloading. The streaming plays well. There is a download button for each video offered and this is the function that is not working on Firefox. The instructions from the web site are to right click on the download button and follow the "Save link as...". saves but not saving as a video file that will play. Other similar subscriptions that I have are working well. Suggestions? Thanks.

Have you tried opening the .html file in a browser, or do you have a plain text editor you can use to view the source code of the HTML file? The file might contain a redirect to a different file, or an error message, either of which could point toward next steps. If you end up opening the file in a tab, you may need to use Ctrl+U to view its source code.