Should we show the sidebar button in the toolbar

Firefox seems to be revamping the sidebar (where bookmarks are shown with Ctrl+B, plus history etc).

I think some of this work is still behind a "sidebar.revamp" preference for ESR 128, which is off by default right now. Part of this is adding a sidebar button to the default toolbar, just after the reload or home button (button icon). But since the preference is off, it won't be shown by default yet.

@donuts I'm guessing the next-next ESR firefox will switch the sidebar.revamp preference, or remove it. We should decide whether we want the sidebar button to appear in the default Tor Browser toolbar. If we don't want it, we can make some further changes to CustomizableUI.sys.mjs.

@ma1 regarding letterboxing, would it be a bad idea to encourage users to use the sidebar, which has an adjustable width?