I’m also joining in to call to not phase out compact mode. It has always been the default mode for me and I’ve been sticking with it ever since the tabs moved to the titlebar, since version 4.0.
Over the years I upgraded from 1280x768/1280x1024 - 1600x900, 1920x1080 and now 2560x1440 and never did I think it’d improve my experience with more vertical space or whitespace in general, and always reset the mode to compact and removed all the whitespace.
It’d be a fat shame if it disappeared, though I have a feeling it will in the next major version bump, it’s marked as [Not supported] after all.
Also a word on telemetry - most people who’d stick to the compact layout probably also opted out of telemetry. I certainly do, and from the complains it seems more people are in a similar situation.
Anyway, all I can do is just add my voice on top of all the other comments and hope Mozilla acknowledges there is a decent user base for this feature.
For now I’ll just stick with the Photon theme while it’s still enabled.