in the meantime, i couldnt keep my hands off this troublesome problem. Although my skills aren't advanced enough to track the problem down, i was able to exclude a few of my assumptions by reverting (through the use of ZFS) all the relevant files to an earlier state (snapshot). By doing this, i was lucky to find a working setup and can now proceed from there.
What i can confirm - as of now - is: The Adobe upgrade did NOT mess up the working state. Neither did the Firefox upgrade from 7.0.1 via 8.0.1 to 9 (performed outside sandboxie) stop the latter from working properly afterwards.
Leaves me a little clueless as to what might have happened, and i may eventually run into the problem again. But since i am warned, i can always snapshot more frequently to see, when and how the problem gets introduced.
At least i can use my "high-level-workaround" even without knowing, what the actual problem cause has been. And to revert to the earlier state lost a maximum of 2 days of work - most likely less.
And, BTW: those enhanced configuration options, together with increased data security had been a major reason for me to "go virtual" in the first place. Windows does not offer anything even remotely close to ZFS.