Firefox, or on mobile, Fennec. It’s a Firefox clone with some added functionality, maintained by the developers of the F-Droid app store themselves, so highly trusted & fully compatible to stay in sync with the desktop Firefox.
For those rare occasions where a website absolutely doesn’t work with FF, and you must use it for some reason, I’d suggest Chromium portable on Desktop, and Kiwi Browser on mobile.
I have Kiwi installed and like that desktop Chrome extensions can be installed on it for the odd occasion. However, IIRC, it is updated infrequently and isn’t recommended as a daily driver.
Last time I tried Mull, I could only use a handful extensions. I chose Fennec particularly because it supports all desktop extensions. Is that still the case?