So honestly I actually quite like the new sidebar layout (I am also a taskbar on the left person so that's probably a factor), but there are a couple of immediately glaring things that could do with improvement:
1. Allow rearranging/toggling/disabling of extensions being shown in the sidebar. I think this has already been raised and it'd be great to see it implemented because it's irritating having random extensions I rarely use glued to the top level of the interface. It would also be cool to be able to put a copy of the jigsaw piece extensions menu down there for neatness.
2. Do something to change how the expand-on-hover interacts with non-tab sidebar icons. I like the hover function (and also appreciate that you can adjust the speed of the animation in about:config, honestly this should be a setting in the main interface settings, not buried in config), but the way it works currently is super irritating, as when you go to click an item such as bookmarks/history/any extension, after a fraction of a second the sidebar expands and that icon suddenly jumps down to the bottom of the sidebar instead. Super annoying to actually click anything as you typically have to move your mouse to it twice.
3. It'd be nice if the sidebar could be manually dragged out to the size desired while still having expand-on-hover enabled, because it would let you see some amount of the name of the tabs without having to constantly have the sidebar fully expanded. Icons alone are slightly lacking if you have more than a couple of tabs open on any given site.
4. Honestly, just make the sidebar as customisable as the top toolbar. You guys have done a great job with how we're able to rearrange and tweak that, and it'd be nice if this was mirrored to any extent possible in the sidebar too.
