Do you have "Sit when away" enabled in Prefs -> General?
There is a bug that causes you to run when standing up from a ground sit.
This bug happens on the LL viewer too, but the LL viewer doesn't have the sit when away option. BUG-10122 - Avatar always runs after standing from groundsit
The next release of Firestorm will have the camera presets feature.
As for the performance problem, there could be several causes.
Firestorm has a lot more options & features then the LL viewer & this can cause some performance degradation, depending on your settings.
If you have dual graphics cards, make sure Firestorm is running on the gaming graphics card. You can see which graphics card is being used under Help -> About Firestorm.
Firestorm uses a higher object LOD setting then the LL viewer & this can cause lower FPS - Preferences -> Graphics -> Objects & sculpts LOD.