Zen Browser
FreePuts the browser's chrome on the side so the page can breathe
An open-source Firefox-based browser built around vertical tabs, workspaces and split views. The interface stays out of the way so the page gets the screen. No ads, no telemetry, and Firefox extensions work.
Why we picked it
Zen is one of the fastest-rising browser projects of the last couple of years: 43,921 stars, MPL-2.0. The problem it attacks is specific — on a widescreen monitor, the toolbar, tab strip and address bar eat a thick band across the top, when all you actually need is the page.
Its answer is to turn the tab strip vertical and collapsible, then add workspaces (keep "work", "study" and "errands" tabs in separate lanes) and split view (two pages side by side, which is excellent when writing against a reference). The base is Firefox, so the extension ecosystem comes along — uBlock Origin and friends install as usual.
The caveat worth stating: it is young. The current version still carries a beta marker (1.21.14b) and releases land close to weekly, so UI and shortcuts can shift between builds. If you want an environment that will look identical in five years, this is not it yet. If you want a genuinely different way of browsing and do not mind adjusting occasionally, it is well worth a try.
If you like the idea but want something more settled, look at Vivaldi — more features, at the cost of not being open source.