To update to 136, go to Help > About. Enabling then consists in enabling 'Show sidebar' in Settings > Browser Layout, and then right clicking on the sidebar and enabling 'Turn on Vertical Tabs'. Make sure to also immediately disable the AI Chat that they offer, because of course they do. Could they have done this in a more reasonable way? Probably.

First thoughts, I have to say I like it. I'll probably find something wrong with it, but I've always found it an interesting layout, I might stick to it for the time being.