The latest version of Universal Control v1.7.4 for your FireStudio Mobile supports OSX Sierra.
You may need to uninstall/reinstall your drivers.
>>> First, download the Universal Control 1.7.4 .dmg file: https://pae-web.presonusmusic.com/downloads/products/dmg/PreSonus_Universal_Control-174.dmg
>>> Use the Uninstaller file from the UC 1.7.4 pkg.
>>> Then click your Go menu and select Go To Folder.
>>> type or copy/paste this precisely.
>>> Click GO
>>> Find the PaeFireStudio.kext file and trash it
>>> Then close the windows currently open
>>> Click the Go menu again and select Go To Folder.
>>> Type or copy/paste this precisely.
>>> Find and trash these files.
>>> Empty Trash and restart
>>> Then run the UC 1.7.4 installer.
If the issue persist, test connecting with a new firewire cable/thunderbolt adapter. If the issue remains create a support ticket from your my.presonus account.