Firefox should use your Download folder (or let you choose a folder) if you choose "Save" in the Download dialog. If you choose Open, it always uses the Temp folder. But "Save" should not use the Temp folder; that is hard to explain.

Acrobat getting into the action is even stranger. Is it possible that a .pdf extension got onto the MP4 file somehow? To avoid Windows hiding such oddities from you, you can set Windows to always show file extensions. See: https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

jscher2000, Thank you for your answer. There wasn't a download dialogue, just a download button on the tribute.co website. Everything else I've downloaded, for years, has gone to Desktop. A mystery. There was no PDF in that filename anywhere and I revised the default programs list and checked the applications list in Firefox; Acrobat wasn't the default in either. Another mystery. The *.mp4 probably originated on an Apple computer, but that shouldn't have caused this. I guess there are some things we are not meant to know.

I didn't know about www.bleepingcomputer.com but I do now. Thank you.