Ok, this is getting strange.
One starting point needs to be included. I did not wish to use my actual zip code for the screen shots (I'm shy.), so I chose a nearby one (60614). I entered that zip into Ff and left my actual zip as a second one.
I began to create the step-by-step list as had been requested. Step 4 was "Select custom". After selecting that, planning to take a screenshot of the image, I moused down to the radar icon on the tool bar to bring up the radar image. That worked, and it worked correctly. It showed the larger image and the unreduced area. However...
At the top of the radar image it displayed "Chicago, IL, 60614, USA... but it showed the map of my actual location... not 60614.
I went back to Options, removed 60614, then moused back down to the radar icon. It showed the same (correct) map, this time with the correct address named at the top.
All I can say is that it's working satisfactorily with the one zip code and that's my main concern. I brought this up, not as a complaint, but only to pass along what I thought might be a bug. I think there's still a bug, but this one is wholly different than the first one I saw.
I hope this helps.