Earlier quoted context omitted.
I don't know exactly what conditions are required to reproduce the bug, but it clearly does not apply to all rMBPs. I'm on a 15-inch 2016 rMBP and have been using Firefox on it successfully since I got it. Before that I spent a couple months on a 2013 rMBP, also no problems. Edit: For people who want to compare notes: High Sierra 10.13.6, Firefox 62.0.2, resolution is set to "Default for display" (which I think does…
It happens when you set your rMBP display to "Scaled" in display preferences, which is what most developers do. (Edit to your edit: Try the "More Space" option in your display preferences! It's amazing.)
Most developers in their early 20s seems plausible, but even there, I surveyed my team’s younger cohort, and it was less than 50%.