@syntax
On the Fdroid forum theres a Known Repositories topic. Lists the 'unofficial Firefox' repo which has various Firefox browsers (including Preview)

I'd recommend using Bromite (also has Fdroid repo). Its a security & privacy focused fork of Chromium which gets fast updates and has built in adblocker, easy to get to site settings to then off JS or cookies, always incognito mode etc.
On Android Chromium has significant security advantages over Firefox.

@friend