Noticed this while triaging bug 1719483 (which is a case where this assertion fails):

void PresShell::DoFlushPendingNotifications(mozilla::ChangesToFlush aFlush) {
  // FIXME(emilio, bug 1530177): Turn into a release assert when bug 1530188 and
  // bug 1530190 are fixed.
  MOZ_DIAGNOSTIC_ASSERT(!mForbiddenToFlush, "This is bad!");

All three bugs mentioned there are closed. Perhaps we can/should take action on the FIXME now and upgrade the assertion to a release assert?

(We do know of at least one way to make it fail -- bug 1719483 -- but in that case, release builds crash anyway, so there's no harm in crashing slightly sooner via this hypothetically-promoted release assertion.)

Searchfox link: https://searchfox.org/mozilla-central/rev/064a1e6a2a6f6aa30be8bf4edea2f8408f779d4d/layout/base/PresShell.cpp#4079-4082

There's also bug 1714290 which happens only with a11y enabled.

Has this been fairly stable for a few years? Maybe it is time to consider upgrading this assert again?

If not, maybe the comment should be updated to point to this bug and remove the reference to the three closed bugs.

We've seen a couple others recently.