App Nap is a function introduced in Mavericks that allows a program not in focus to use fewer resources. On Yosemite, this function causes a fullscreen FS to time out, forcing a disconnect. You an solve this by disabling App Nap, as follows:
More info on what App Nap does can be found here.
If you are using Firestorm 4.7.1 on Yosemite, and have an nVidia graphics card, you are likely experiencing this issue. It appears that Apple decided to include an older version of the graphics card driver for this release.
No solution is available as of the time this was written; please refer to the JIRA linked above for the most current information.