F-Droid wants to drop Firefox because Google Play is now required to build it (though not to run it). Same reason why Chromium isn't being considered:
https://f-droid.org/forums/topic/chromium/page/2/#post-16388F-Droid has (temporarily?) taken down their Fennec fork, seemingly due to difficulties with development: https://f-droid.org/forums/topic/please-do-not-drop-firefox-...
This is presumably why Firefox's removal has been continuously postponed. Reliability is one of the reasons why I would prefer first-party Firefox over a third-party fork.
Though there are good reasons why F-Droid doesn't include Chromium and seeks to remove Firefox, the absence of many open source Android apps from F-Droid's catalog has a direct impact on utility. It's also not uncommon for apps on F-Droid to lag behind the version on Google Play or the latest release direct from the developer. (Again, there are good reasons why they screen apps, but it does impose a delay.)
I appreciate F-Droid's screening process, and I wouldn't want it to go away, but it'd be nice to have an alternative open source Android app catalog that's a little more liberal.