I'm wondering if she was driving with the windows rolled down.
Simplest thing to do would have been to drive alongside her and toss an open container of gas thru the window along with a cigarette.
I've started a new thread for today's article on the Fire Department's finding at the time of the fire:
Car fire set off suspicions as crew fought it (Smith Tennesee License Scandal)
They are comparing it to some older cases they've seen (could even indicate the work of someone like the Dixie Mafia with an established presence in the area).