steam_api64.dll is part of the steam client and manages api connections with Steam for things like authentication, game updates and cloud storage. Weird that removing that solves the issue - I wonder if it's trying to update something on Steam and is unable to do so (i.e. that file becomes locked by a process).

Do you have anti-virus software running on your computer? If so it may be worth excluding the FF folders/Steam folders as it could be this locking a file and preventing it from exiting properly.

It's also worth disabling any overlays (where possible) you may have active (steam, nvidia, etc) as these can also cause issues.