As a reminder, Brave was created by the guy who brought you JavaScript and was later fired from Mozilla for donating to hate groups. Brave also profits from multiple forms of fraud including NFTs and affiliate hijacking.
If folk want to have a chromium-based browser made by a company, take a look at Vivaldi instead (which will keep the old plugin architecture, so adblockers work). It has a limited built-in blocker and extra features, but for now still runs uBlock.
Vivaldi is what I use, and it's absolutely the best Chromium browser I've ever tried.
That said, I'd switch to Firefox in a heartbeat if it could duplicate that sidebar. I use that thing all the time, and it's the only thing keeping me on Chromium.
As a strictly Firefox user for... as long as I can remember... what side bar? If it's a bookmark or tab container I could probably hack a plugin together pretty quick.
Opera chat-tabs. Opera has a sidebar for chat clients (eg messenger, WhatsApp, telegram,...) built-in. It works really well and is prettier than the Vivaldi implementation. Still sticking with Vivaldi though
It's a thin, persistent bar on the left side of the page with tools and shortcuts. By default, it's where you find bookmarks, history, downloads and things like that, but you can also add custom websites to it as well. They pop out either over or alongside the main window (depending on whether the pin button is pressed), and they display the mobile webpage when available, to be more usable in such a small window. It's how I use Discord and Mastodon.
As A Vivaldi user since day one, I gave it up once I couldn't take anymore of Googles/Chromium bull hockey. I'm just so done supporting a company like that. Firefox switch wasn't easy, but after the first week I got used to it and I'm never going back to Chromium browser. Anyone can switch if you actually care enough.