Just in case you do not know, or you are interested, the Firefox's UI is mere CSS, one can customize it through "chrome/userChrome.css" in the profile folder of the user. I put for example my tabs under the bookmarks, just above the page content (As FF used to be at past), with the template from the CustomCSSforFx[1] project, and modified some colors and icons.

Firefox have also the "Browser Toolbox"[2], homologous to the web development tools, that one can use for to explore the css of the UI, select elements, etc, and modify them for to see how would look the modifications before to put them within the userChrome.css

The tabs I think that are under the id=TabsToolbar, so an #TabsToolbar{ display: none !important; } or similar should work, in addition to correct with other css params the space that it leaves.

Note: What happens is they change things from time to time, and after an update one have to correct the css again, I warn.

[1] https://github.com/Aris-t2/CustomCSSforFx

[2] https://firefox-source-docs.mozilla.org/devtools-user/browse...

Firefox also doesn’t let you hide the gigantic sidebar header without usercss hacks.

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.