This is not a malware question so I may not be the best person to help you but I will have a go.
I am wondering if you are updating an old version of Firefox because when I search for a solution I find this:
Mozilla has not had .zip for Release ever since Firefox 1.0.2 was released (Mar 2005) as mixing the .zip and setup installs on Windows caused bad crashes.
Try uninstall every thing related to Firefox/Mozilla and then download and install a fresh copy.
To remove your Firefox fully you need to remove profile data and settings. Before doing this you may want to backup your bookmarks.
To back up your bookmarks:
In Firefox click the Bookmarks button Show All Bookmarks (likely down the bottom) > click Import and Backup(toolbar along the top) > Export HTML... and save it to your desktop.
Later when you re-install FF you can reverse the process and Import HTML... when the Wizard comes up just import the HTML file you had saved earlier.
Next
Please go to Uninstall Firefox and follow the instructions for uninstalling Firefox. Make sure you check the box to Remove my Firefox personal data and customizations.
Download and install a fresh version from Mozilla see link below:
https://www.mozilla....US/firefox/new/
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