@kgslotkow wrote:"Hmm, did it add anything that mentions MPEG or video?" No.
"If you try the download again, does it open automatically this time?" No. With Firefox version 85 and earlier, I was able to click on a MP4 file in my email account and it would open automatically in the Windows Media Player app. With version 98 and later in won't!
It's not automatic. As currently designed, you need to use "Always Open Similar Files" on the downloads list to set it up. Immediately after you click that for an MP4 file, does it ask you what application to use, or does it use Windows Media Player automatically.
"As for why the list changed, I don't know." The content list has been significantly reduced! Here is a partial list of Content Types (extensions) that Firefox version 98 and later will not open automatically: CAB, CSV, DWG, JPG, XLS, DOC, MP3, MP4, MSI, PNG, Quick Time, Text Doc, TIFF, and XLS. I wonder why the developers/programmers at Mozilla would eliminate them? Is this suppose to be better? Who at Mozilla can I contact to inform of these deletions? Or can I suggest to someone to have a way of adding file extensions to the Contact Type list? The newer versions Firefox in my opinion has been downgraded.
I don't think any of those were built in. You may have added those items over time using the old method: Firefox would display a Download dialog with the choice to Open the file in the system default application (or whatever application you prefer) and there was a checkbox to do that every time. That checkbox morphed into "Always Open Similar Files".
Starting in Firefox 101, due out in a couple weeks, you'll be able to set Firefox to go back to the earlier approach of showing the Download dialog for unknown content types. Here's a preview based on the Firefox Developer Edition (beta test release):
