Found a bug with the personal extinguisher today after testing. I started with a saved game that had my FF needing to upgrade (again). Saved after upgrading, saved as... and logged to main menu. Logged back into the new saved game, and sure enough, the extinguisher needed to be upgraded again. Loaded the first save, upgraded again, and played out for several hours, checking after each fire and such, and never needed an upgrade. Checked one last time (fine), saved, logged out and back in again, and sure enough, I needed an upgrade.
Basically, when your game saves, it is NOT saving the upgrade.
For reference, the upgrade I refer to is the one that gives the orange discharge from the nozzle.