Studio One v2.6.5 was the last release for Studio One 2 that runs on Mac OS X 10.8, 10.9, 10.10 and 10.11.
Now when you say FireStudio, you have to be careful because there are several products with that name and one of them does not work on 10.8 or higher, which is the FireStudio (2626) last supported OS was: 10.7.5 with Universal Control 1.2.
If you have one of the other FireStudio interface products, their compatibility is as follows.
FireStudio (Project or Mobile) as well as StudioLive Classic Mixers are supported on 10.11, 10.10 with UC 1.7.3
FireStudio (Lightpipe and Tube) which are no Legacy Devices are also supported on 10.11 and 10.10 with UC 1.7.3
Updates can be found on the download page: http://presonus.com/support/downloads