Disable a specific app from ever bouncing in dock
Solution 1:
It's all or nothing with the Dock. You have two options: disable bouncing for anything and everything by entering defaults write com.apple.dock no-bouncing -bool TRUE; killall Dock into a Terminal window, or modify & recompile Filezilla, which is not something anyone would want to do I guess. (A third option would be to get a new FTP client. Honestly, Cyberduck is really good.)