Believe it or not, this is a bad idea.
Here's a pretty useful lifehack: do not set off fireworks mortar shells inside your house. I know this seems counter-intuitive, but please take my word for this.
Hey, I understand your predicament—you want to set off a terribly dangerous explosive device, but you're not currently in the relative safety of the outdoors. What are you gonna do in a situation like that? The answer to that question is: stop moving the lighter in the direction of the mortar shell fuse, and go outside.
Another, possibly more useful, lifehack: do not set off a fireworks mortar shell on your head.