The first leg will now take place at 16:45 (UK time) at the Academy Stadium - 15 minutes earlier than previously scheduled - on Wednesday 3 March.
Meanwhile, the trip to Italy will start at 13:00 (UK time, 14:00 CET) on Thursday 11 March - half an hour earlier.
The pair have never met before in professional competition.
Semi-finalists in 2017 and 2018, City reached the last 16 stage with convincing victories over Göteborg FC in the Round of 32.
Georgia Stanway's 50th goal and a Sam Mewis header on her Champions League debut secured a hard-fought away win, and City cruised to a 3-0 home triumph in the second leg, thanks to a Lauren Hemp wonderstrike and Stanway brace.
Fiorentina will be hoping to reach the quarter-finals for the first time.
The Italian side eliminated Slavia Praha 3-2 on aggregate in the Round of 32, thanks to Daniele Sabatino's late strike.
The tie will see Dutch midfielder Tessel Middag face her former Club.
The game will be available on CITY+*, while BT Sport will also be showing the games.
*Excluding Israel, Australia and Mexico.