How can Firefox possibly improve if every solution gets labeled a hack?

I’m not sure I follow? The clear improvement here is to add a checkbox representing a supported option to hide that header.

Hiding it in usercss isn’t supported and is liable to disappear at any point (say, the developers decide they’re tired of maintaining some part of the code that makes the custom usercss flag work). It serves the purpose for now, but cannot be relied upon in the long term and as such is a hack.

Not a Firefox user, but this feels like a culture gap. I use multiple applications where configurability through user-code is part of the SLA, and dropping support would be as utterly unacceptable as, say, dropping the toolbar. Considering the contrast Mozilla is trying to make with Chrome's more restrictive and non-user-conscious nature I see no reason they wouldn't share that same philosophy.

Unless there's something I'm missing re: Mozilla's history/market-positioning?

In the past they’ve not hesitated to reduce customizability when doing so is perceived to bring some benefit to ease of development.

Aside from that I just think it’s a good idea to push for implementation of basic functionality like hiding bars that don’t pair nicely with popular vertical tab extensions. I shouldn’t need to resort to usercss mods to do something that simple.