Did you try it after Firefox is already running in sandbox?
No I didn't, I had gathered (perhaps incorrectly) that it worked ok if Firefox was already opened. Maybe I got that in reverse? Anywho I'll try to get a trial of the full DC [not just reader version] and see if I can reproduce it with that one. I'll be sure to verify firefox is already running before trying this time! Sorry...
Update:
Confirmed the issue when firefox is already running sandboxed.
SBIE2205 Service not implemented: DDE DATA
It doesn't happen if firefox hasn't been started. It's the reverse of what I saw last time I looked at office, odd! (unless I'm just recalling incorrectly)
I haven't seen any easy workarounds yet but I'll let you know if I come up with any possibilities.
A weird thing I noticed is that it does still seem to open the page inside the sandboxed firefox, on occasion though it is a bit later, if I quickly
double-tap the link in adobe dc.
Update2: So it seems I had it wrong, it's once I press 'OK' in the first Acrobat error, "The web browser has not responded to your request for over 1 minute: terminating the request." that it then opens the url in the already opened sandboxed firefox. Funny thing is it's not a minute going by, just a few seconds before that alert pops up. I'm going to throw out a guess that it's due to some communication blocked by SBIE (or not accounted for in adobe) [and not showing up in resource access monitor, I only saw Sandboxie_DDE_ProxyClass show up] which ends the attempts early. I also tried looking through various registry entries and tried to eliminate any /dde entries similar to what worked in office but nothing changed in my attempts so far. In short, I haven't really made any progress and unless I can suddenly think of a new approach to try it looks like we'll need to leave this to the pros to figure out what is going on and if it is an issue with SBIE or if it can be
worked around on SBIE's end or not.