See also: Ungoogled Chromium, Vivaldi, and Chromium in general.
Firefox woes
Firefox has dramatically disintegrated under the so-called leadership of Mitchell Baker, enriching herself to the tune of millions while the market share (and revenue) of Firefox plummeted and 250 employees were laid off. This alone is reason enough to avoid such a toxic software environment. There is no need to coddle multi-millionaires in order to save open-source projects, which they won't do in any case, by definition.
Firefox removed it's promise not to sell personal data.
And frankly, Firefox performance has not for years and years been faster or more stable or more interoperable than blink-based browsers. Sad, but true. Let's not kid ourselves.
Yes, there are lots of reasons to not use a Chrome-based browser if one wants "true" open source, whatever that might mean. For me, it comes down to Ungoogled Chromium on the desktop and Vivaldi on the browser. Yes, as far as I can tell, Vivaldi is open source, though it also uses licensed code.