"Note that Iceraven Browser includes some unstable code written by Mozilla, with our own added modifications on top, all shipped with the stable version of GeckoView engine."

I wouldn't use iceraven if security was a priority.

Fennec on the other hand is much closer to FF , quoting their page :

https://f-droid.org/packages/org.mozilla.fennec_fdroid/

"Fennec F-Droid is based on the latest Firefox release (codenamed Fenix).

It has proprietary bits and telemetry removed, but still connects to

various Mozilla and Google services that can track users."

Stability and security is not a priority though. Updates are slow.

I use it primarily for compatibility with my favorite add-ons not available on FF Android: I don't care about cookies, Cookie AutoDelete, Translate Web Pages, etc.

Mull is another FF Android fork may be worth taking a look at.