I see the same now and then with Firefox, Edge and other browsers, unfortunately there's too many variables to do other than speculate the number of connections a signal has to go through is getting temporarily broken somewhere along the way. Most times a download can be resumed without issue and complete, sometimes have to just wait and try to resume again. If a download finishes properly it pretty much rules out hardware issues. Then I have to keep in mind the days of the '90s and Dial-Up connections of 53Kbps at best but usually slower.