The 14th level Circle of Wildfire ability reads:
If the spirit is within 120 feet of you when you are reduced to 0 hit points and thereby fall unconscious, you can cause the spirit to drop to 0 hit points. You then regain half your hit points and immediately rise to your feet.
The Instant Death rule states:
Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.
If a single attack did enough damage to cause Instant Death to the Wildfire druid, could Blazing Revival still keep them alive and restore them to half hitpoints, or does the Instant Death trigger before that is possible.
Along the same lines, what about Disintegrate? (I can move this to a separate question if that's preferred)
Admittedly, this scenario should be unlikely for a 14th level character unless they have a really bad constitution score, or take an absolutely massive crit.