Turned daylight cycle off in new world (large biomes - swampland), toggled between firetick being on and off a few times, found that in neither Creative, Survival or Adventure fire does not visibly spread to flammable blocks regardless of firetick setting, but flammable blocks do disappear. Enabled daylight cycle and flammable blocks burned in some instances but not in others.

To recreate:
1. Disable daylight cycle
2. Enable firetick
3. Set down flammable block
4. Light fire near it
5. Block will disappear
6. Enable daylight cycle
7. Retry 3 & 4
8. Some blocks will burn away, others will disappear.