Autosandbox in avast will only run if a suspicious program detected by avast.
Sandboxie is completely virtual by enable firefox to be fully virtualized in sandboxie settings.
Btw. I even disable the add ons but the FF is crashing.
Added.
Based on my experience i have encounter with avast autosandbox.
It will only activated if a program is suspicious.
On default Firefox is already excluded in filesystem shield which is autosandbox rely on it... I say this because one of the game im playing is detected as suspicious and autosandbox always poping up.
I did try excluding on autosandbox setting but it still detecting suspicious.
What i did is excluding the game on filesystem shield and no more suspicious detected.