(My) original report on Launchpad: https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1820514

Any page under https://forums.tomshardware.com/ seems to trigger this, but I have yet to come across other sites that do. Pages can even be saved from the main Tom’s Hardware domain (https://www.tomshardware.com/) without issues.

A workaround discovered by Olivier Tilloy: clicking retry in the download manager completes the save.