I have found a reliable and replicable way to exponentially increase or decrease the fire burn time which I can only assume is a _bug_. (Why does this insist on putting _s around the word _bug_?)

Here are the steps:

---Initial setup
Set Game to 'slow' mode
Gather Firewood to max fire
Time remaining for fire: 33h 19m

---Start Repeat to increase fire time---
Spacebar pause
---Menu switch to fast thusly:
Menu button, Options button, Fast option, Done button, Play button
Time remaining for fire: 33h 19m (STILL!)
---Menu switch to slow thusly:
Menu button, Options button, Slow option, Done button, Play button
Time remaining for fire: 111hr 01m (!!!)
---Repeat to increase fire ridiculously---

---Setup to 'undo' the stolen burn time
---Menu switch back to fast thusly:
Menu button, Options button, Fast option, Done button, Play button
Time remaining for fire: 33h 19m (Back to it)

---Start Repeat to decrease fire time---
Spacebar pause
---Menu switch to slow thusly:
Menu button, Options button, Slow option, Done button, Play button
Time remaining for fire: 33hr 18min (Still.)
---Menu switch to fast thusly:
Menu button, Options button, Fast option, Done button, Play button
Time remaining for fire: 9h 58m
---Repeat to decrease fire---

Note: This doesn't seem to affect potion evaporation time (unfortunately) or (thankfully) nursing time.