How to run iD locally with background imagery and OSM geometry?

I have been following a PR to iD that implements a feature I asked for a while ago (auto-collapsing the sidebar on narrow screens).

I managed to test the changes locally by following the instructions in the README, but although I get a working UI, the actual map shows up as completely black: no background imagery, and no OSM geometry:

I did make sure the “live” button in the bottom right corner was toggled on, as suggested in

Is this behavior normal? Are there any steps and/or setup I could do to be able to actually see something in the map when running iD locally?

Answering myself: after testing some more, the issue seems to be with Firefox. I can successfully run iD in Chomium-based browsers, but Firefox shows a black area where the map should appear, even when running in safe mode (i.e. with all add-ons and themes disabled). If anyone else is able to replicate the issue, it might be worth adding the caveat to iD’s contribution guidelines.