I have used FF based browsers for a long time and still do. I recently saw this from the GrapheneOS developers, which kinda freaks me out and has me considering switching to a Chromium based browser:

https://grapheneos.org/usage#web-browsing

Chromium-based browsers like Vanadium provide the strongest sandbox implementation, leagues ahead of the alternatives. It is much harder to escape from the sandbox and it provides much more than acting as a barrier to compromising the rest of the OS. Site isolation enforces security boundaries around each site using the sandbox by placing each site into an isolated sandbox... Browsers without site isolation are very vulnerable to attacks like Spectre...

Avoid Gecko-based browsers like Firefox as they're currently much more vulnerable to exploitation and inherently add a huge amount of attack surface. Gecko doesn't have a WebView implementation (GeckoView is not a WebView implementation), so it has to be used alongside the Chromium-based WebView rather than instead of Chromium, which means having the remote attack surface of two separate browser engines instead of only one. Firefox / Gecko also bypass or cripple a fair bit of the upstream and GrapheneOS hardening work for apps. Worst of all, Firefox does not have internal sandboxing on Android. This is despite the fact that Chromium semantic sandbox layer on Android is implemented via the OS isolatedProcess feature, which is a very easy to use boolean property for app service processes to provide strong isolation with only the ability to communicate with the app running them via the standard service API. Even in the desktop version, Firefox's sandbox is still substantially weaker (especially on Linux) and lacks full support for isolating sites from each other rather than only containing content as a whole. The sandbox has been gradually improving on the desktop but it isn't happening for their Android browser yet.

EDIT: I really hope Ladybird turns out to be amazing.