To sum-up, this particular bug might be a show-stopper for some change password forms - like the ones that do not throw proper error messages and an annoyance in the cases in which the password form does have proper error messages. Further more, even though it's not necessarily a typical edge case, it needs a few requirements to get triggered. (see also https://bugzilla.mozilla.org/show_bug.cgi?id=1570319#c0);
My understanding discussing this bug with Matt was that this might be tricky to fix without breaking existing password only functionality and that further thought is needed on it. Strictly from the QA perspective, we'd like to have this bug fixed for Fx70, but wouldn't force a risky fix.
I'd guess the above should explain why this bug is P2/normal at this moment.