For starters, I am absolutely delighted by the "brief history of the fire engine" oh my goodness.
I don't know if you were going for Discworld but that's what I'm getting from the setting, which is a compliment from my perspective and I hope from yours. Actually I think you might have a book/story in here too if you wanted!
Character sheets are wonderfully done! I love how much personality is just in the poses alone, so many props to your artist, wow. And the sheets aren't overwhelming, which two page ones can easily be.
It's interesting that you give the disclaimer early on that people need to be familiar with games already because you've done a really great job at breaking things way down to be very accessible to new gamers. The scene by scene breakdown I can see being especially great for new GMs! I'm picturing handing this off to a determined teen or young adult to host at a family gathering because they're really into Dimension 20 and want a starting point.
Flashbackdrafts are such a solid pun on top of being a great way to allow rerolls.
I had two suggestions and both feel like nitpicks with how much time you've put into this:
I would love to see what your vision for the fire engines actually looks like! Unless I missed it somewhere I don't think any of the wonderful illustrations featured the actual Lexie in full and I feel like it would really add to it, though I understand something that complex is a lot to ask of an artist.
I know this feels like a wild thing to say about a 56 page document but I do feel like the text needs more room to breathe in some places, especially in the earlier pages - all of Safety on its own page, all of Characters on its own page, etc.
Basically I just feel this is a really great, really thought out game and just another pass of polish could make it perfect! This is definitely going in my "to play" pile.