Hi.
Like the title says, playing the firefly festival, and its my first gig at a festival of this size... Doing it on a budget, and my plan is bringing a 110w bass amp with a 12" speaker as a stage monitor, running everything else through the house, through a sansamp DI... This has worked for me at 2,000+seat theatres just fine, and I am wondering what, if anything, sounds crazy about that to all you guys on here. Thanks.
It'll be enough as a monitor for
YOU and only
YOU if the manure hits the propeller and the monitor guy is struck down by a lightning strike.
Or otherwise "engaged" if You know what I mean

.
In any case, all Your fellow band members will be at the mercy of the monitors.
If they're fine with that, then all is cool.
If not...
Unless You're the headliner (and sometimes even then), festival gigs tend to be very hectic, very little time to change the gear, nerve-wrecking for the unprepared and a disaster if someone has pissed off the sound crew.
The simpler and easier to move Your rig is, the better.
Something on a cart or a dolly, with only one power cord and the DI to plug in that can be rolled on and off the stage quickly, will go a long way towards a successful gig.
And the invitation to come back again next year

.
Some engineers will insist on their DI, some insist on mic'ing, some do both, almost everyone will use whatever You want to use if they have ample time to make the needed changes (the sound-check is way too late BTW).
You probably have recieved the festivals instruction package with the contact information where to send the stage chart, what's the monitor, backline and sound crew compliment with the necessary details on the other logistics involved, what does it say?
Regards
Sam