Click on Responsive Design Mode, Choose the Edit list for devices. The custom devices are cut out on the right side. Tested Firefox Nightly 58.0a1 (2017-11-02) (64-bit)

Hmm, that is quite interesting... I can't seem to reproduce it here on macOS, which it seems like you are using? Do you see the same thing in Beta / 57?

Let me check. Nope on Firefox 57, the custom list is not in a separate column, but under watches. I just realized the are can be scrolled vertically or horizontally, but it's a bit weird.

Is there a way to inspect the DOM of this pop up?

(In reply to Karl Dubost :karlcow from comment #3) > Is there a way to inspect the DOM of this pop up? Yes, you can use the Browser Toolbox to reach the UI like the device modal here. https://developer.mozilla.org/docs/Tools/Browser_Toolbox (In reply to Karl Dubost :karlcow from comment #2) > Nope on Firefox 57, the custom list is not in a separate column, but under > watches. > I just realized the are can be scrolled vertically or horizontally, but it's > a bit weird. Very curious. I am aware it somewhat awkwardly scrolls vertically (because it is a fixed size, and the phone column is now too long), but the horizontal scrolling is new to me in this bug, so hopefully we can find the cause!

Okay, I see what's happening now. Once you have at least 7 custom devices defined, it wraps to the next column. I expect past versions also behave the same, as long as you have that many custom devices.

Comment on attachment 8926583 [details] Bug 1414159 - Extra modal width for RDM custom devices. https://reviewboard.mozilla.org/r/197818/#review204010

Pushed by jryans@gmail.com: https://hg.mozilla.org/integration/autoland/rev/0dc2628668b2 Extra modal width for RDM custom devices. r=gl

Mass won't fix for 58. Let it ride the train.