re-install your browser

As far as I know Firefox has never 'warned' about a duplicate file being saved. If the dupe would happened to be saved to the same folder, Firefox appends the file - as such - image(2).png or Firefox Setup 40(2).exe; at least with Windows OS currently. If anything, that would be a function of the operating system, AFAIK.

And as I recall happening with Ubuntu-based Easy Peasy Linux on a EeePC netbook that I was using a few years ago, the same 'append' to the file / image name when a file by that name was already in the folder being saved to. If saved to a different folder the file name was saved as downloaded = no 'append' happened. It seems to me that the operating system is the 'one' doing the recognition of file name already being present in the selected folder, because I have seen that 'append' happen when doing hard drive 'maintenance' or re-organization when I come across files that I saved to the 'wrong' folder initially and I went to move a file to the 'correct' folder = message that the file already existed in that folder and the 'drag' operation was cancelled.