If uploading a .gpx file (list of GPS track points - in XML format), FF will open a tab then another etc quite quickly. It also creates a 0 byte file in the downloads directory. This happens eg if using the Google maps to gpx converter (https://mapstogpx.com/). If FF is closed down, on restarting it just continues - infinite loop. The way to stop is invoke FF by clicking a saved link or from command line with a url as argument. This stops the loading of the gpx as your chosen page is loaded instead. You can then hover over the existing GPX tabs and select remove duplicate tabs.