Maybe if you provide relevant information like what happens when you try to download a zip file that would explain a little bit the problem. You do what you want but without explanation it is all a guess game, it’s not helping you.
I have no extensions installed - I am just using Firefox as it is shipped. The only change I made was to use another skin supplied by Firefox - perhaps resetting to default skin will resolve the issue.
Update - switching back to the default theme fixed the issue for me, thank ishaanbhimwal for your time and help.