I looked at the Universal fireworks, looks like they are at 9:45, so if you heard them closer to 9:00 that may indeed be Disney. I guess SeaWorld could also be doing a test but you're fairly south and west of them too.
Also looking at the times it looks like Magic Kingdom closed at 9, Epcot at 11, and HS and AK at 8. Assuming they don't want park guests to see the testing (and you can't really hide fireworks in the sky) I would assume they weren't at MK or Epcot. I'm sure it would be all over social media if it was. AK doesn't do fireworks because of the animals, so the best guess is HS. Maybe they are testing to bring back Fantasmic or the Star Wars fireworks? I think I've heard rumors of Fantasmic coming back, and I know they would want to do full rehearsals with the cast and pyro to make sure everyone is 100% safe. I know people and companies can occasionally rent out the parks and have private shows, but I don't think they are doing that now.