Some other browsers have a thin sidebar with icons that allow you to open internal browser pages (bookmarks, history) or websites themselves (mostly in a mobile layout due to the small size). With some even having extra functionality for messengers.
Sure, Firefox already has a sidebar, but you open it from the toolbar (you can change to other pages from within the toolbar, but it's done via a dropdown menu that eats up space from the sidebar), you can't keep it open and "minimize" it to only show the icons of the pages that can be opened, and only extensions are allowed to open or add pages to it. I don't think it currently sees much use natively either, as even the default Proton layout has it mostly hidden unless you use an extension that makes use of it or get the sidebar icon back into the toolbar.
Overall it's would be an excellent productivity tool, as you could use it to quickly look up the term you have highlighted without having to leave the page, pin a dictionary site to it and look up definitions as you are reading an article or to read your mail or new texts as you are working on the main tab.
Extra integration with popular messengers to get new message notifications from a pinned site even if the site isn't currently open in the panel or the ability to drag toolbar icons into the sidebar would be nice to have, but probably not strictly necessary either.