>Faster, less bloat
Stock Firefox doesn't actually have that much bloat, and it's not noticeably slower than Chrome on reasonably modern hardware (i.e.: most page loads are near-instant, same as in Chrome on a good connection).
>Extension support for Chromium is way better too
Until Google decides that your extension is unworthy of being in their store. This notably happened with Pushbullet quite recently: https://news.ycombinator.com/item?id=23168874
... Until Google inexplicably restored it a few days later, but not before lots of accusations were thrown around.
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IMO it's also worth noting that ungoogled-chromium is (obviously) an unofficial fork of Chromium. Google may at any time change Chromium so substantially as to either require Google integration at some fundamental level for even the most basic functionality, causing too much work for such a low-profile effort to continue, or just make Chromium closed-source. With Firefox, that risk doesn't exist because of the business motivations of the company that develops it.