So I read that sitting on a toilet for too long is not good for you and can make your fissure get worse.
But what about on a chair?
I'm working at a job that pretty much makes me sit in a chair for like 8 hours a day.
Would that slow down or stop the healing process?

I'm in a similar situation and in the few weeks I've been dealing with this I've found sitting too long makes it angry.

it gets angry? yikes!

So did it just flare up again and caused pain or something else?

Yeah it seems like especially for the 5 or so hours after a BM, the fetal position is safest for avoiding spasms. Sitting or standing more than a few minutes will often start to hurt, depending on how good of a day I'm having.