just use the .snap package (they have all firefox channels as snaps stable, candidate, beta, edge)

channels: stable: 64.0.2-1 2019-01-10 (167) 216MB - candidate: 65.0-2 2019-01-28 (172) 217MB - beta: 66.0b3-1 2019-01-28 (173) 217MB - edge: 63.0b13-1 2018-10-09 (140) 206MB - esr/stable: 60.5.0esr-2 2019-01-28 (171) 213MB -

snap install firefox