My normal setup is to plug my Macbook into two external monitors, and leave the clamshell itself closed. and then when I travel I disconnect it and just use the built-in monitor.

Lately I’ve been running into a problem when I open up Firefox on the Macbook, after disconnecting from the external monitors. Everything is twice as small as it should be. Resetting the Zoom feature didn’t change anything; it wasn’t just the page elements, but the browser chrome, settings dialogs, etc. Everything had shrunk and was barely legible.

I figured it was related to the Macbook’s retina display, and poked around about:config until I found the culprit. Changing layout.css.devPixelsPerPx from 1 to 2 fixed the problem.