Firefox has telemetry, it just doesn't gather data on this feature. It does gather:
Interaction data includes information about your interactions with Firefox such as number of open tabs and windows, number of webpages visited, number and type of installed Firefox Add-ons and session length, as well as Firefox features offered by Mozilla or our partners such as interaction with Firefox search features and search partner referrals. [0]
I don't think telling if the compact UI was enabled is more invasive than this data.
I opt out. I assume people inclined to tweak their FF settings are more likely to opt out than those who don't. By definition, I guess. So that's gonna spoil whatever statistics you want out of this.
[0] https://support.mozilla.org/en-US/kb/telemetry-clientid