Firestorm comes with built in pose stand functionality. You can access it from the top menu, Avatar → Pose Stand.
NOTE: There is a tutorial video on this feature here.
This opens a small window, like the one shown here. You can use the drop down list to select one of several poses for your avatar.
When done, simply close the Pose Stand window and you will be released from the pose.
Note that no pose stand is actually rezzed; none is needed.
NOTE: The pose stand animations will override those in your AO - unless your AO animations are very high priority. So if you find that you are not being posed when the Pose Stand window is open, then turn off your AO.
If you click the check box at the end, beside the lock symbol, you will be forced into a ground sit. This acts as a move lock function, and thus replaces the need for scripted movelock gadgets.
NOTE: The pose animations will not play on grids other than Second Life.