I just ran into this myself. Here is how I fixed it. (Found this on a previous post)
The System Manager config file is in User preference folder.
All the stored details can be edited at "SystemManager.config" file present in the folder:
Worked in Windows 8
Find the Folder
Edit the SystemManagerconfig file ** (I used Notepad+ to open)
Change the path to firefox "C:\Program Files (x86)\Mozilla Firefox\firefox.exe"
Save and close
Hope this helps. 🙂