Firefox stores any incomplete download data in a .part file and basically renames it without the .part once it's finished downloading. It's safe to delete it if you've canceled the download or if it's still there for some reason after finishing. If the internet cuts out, delete the empty temporary non-.part file Firefox made and rename the .part file without the .part extension, it should play up until where it stopped downloading.
VNs are superior to anime, don't @ me