About 2 months ago i posted a topic asking this very question. The websites i had found claimed it could reach 25mph just like the other shermans.
I was told tho that those websites were wrong and the slow speed was due to extra weight put on by a new turret to house the 17pnder plus armor.
Edit: Plus i think someone said the firefly didnt have the same type of engine the other 2 shermans had. the engine it had, had less horsepower.