I've been making assumptions about the name of people from Jacksonville, but I have yet to look it up. In fact, I will wait until I offer my best guesses before I actually look for the real answer. I know I am now a Floridian (which means I need to drive around shirtless more often), but I'm still trying to figure out my city and county name for myself.
I'll base my guesses on past experience and similar places. I was once a Milwaukeean, both city and county. And a Wisconsinite. I think I was a Lenexan in Lenexa, Kansas, as well as a Kansan. So those were easy. Actually, I'm not sure if counties tend to have separate names for residents. Milwaukee was the same, but do the people from Johnson County, KS, call themselves Johnsonians? OK, besides the point.
I now live in Florida, Duval, and Jacksonville. Am I Duvalese? Perhaps Duvalean, Duvalan, Duvalite, Duvaler,
Jacksonvillean? Jacksonvillanese, Jacksonvillian, Jacksonvillite, or Jacksonviller, Jacksonvillains, or Jacksonvillan?
I saw Jaxholes and Rednecks on Reddit, but those don't seem official.
Apparently, local news outlets reported recently that people here are called Jacksonvillians and Jaxsons (Jaxons). The latter is a bit like people from Saxony, which might make you feel more Continental. But I'll probably stick with Jacksonvillians.