There are some cases when Firefox initiates a restart:
1. Updating Fx prompts to restart it
2. Installing non-jetpack add-ons requires browser restart to run them
3. There are some add-ons like "restart button" that add a button that restart firefox upon clicking it.
4. Crash causes crash reporter to pop-up. Crash reporter always fails to submit a report and suggests to choose 1 of 2 actions: either close Firefox or restart it. User may choose to restart it.
In all the above cases firefox will restart without firefoxportable.exe, which will make it run in non-portable mode.
Could you do something to fix firefoxportable behavior for all (or at least some some of them) the mentioned above cases?