Sure you can do this - the answer is staring you in the face on the MPW prompt window
- and you have two options, actually...
1. Save MPW to memory - this is a per session save, so after closing the last FF window, then opening a new window, you'll again be prompted for the MPW, which will again be saved until you close the last window...
2. Save MPW to disk - this survives sessions, and reboots as well...
Of course, both of these bring a higher security risks with them - the safest is to never save it at all...