Created attachment 8522208 [details] nightly-profman.png See screenshot. I'm working on a 15" rMBP with a 27" Dell monitor attached. When started with -p, the profile manager starts on the large (non-Retina, non-scaled) screen. See also bug 1098299, which might be related.
Flagging for backlog, although I suspect this is a core bug of sorts...
Component: General → Startup and Profile System
Product: Firefox → Toolkit
Flags: firefox-backlog? → firefox-backlog-
Created attachment 8739380 [details] profman-45.png Another data point: Similar setup as OP's (Macbook Pro 15" retina + 27" 4K Dell), starting Firefox with -P, but in my case the Profile Manager appears on the laptop screen. This is with Release (45.0.1), but I've seen it happen in previous versions.
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