When updating the firmware of anything, whether it be cameras, pc'c, Nas boxes, etal, it's always best not to use remote updates, yes, they do work, but not guaranteed to. As for downloading files, I used to use all sorts of browsers, but non work as good as good old MS Edge.
So, my method, which works every time is this:
Download via Edge, the firmware update
Just use one SD card, for me Sandisk cards always work, and format it in camera
Take the SD card and place it in a SD card reader attached to a pc, copy the downloaded firmware file FWUP0027.DAT to the SD card
Place card in camera, and do the necessaries
For me it works every time