Upvote:1
According to Spirits website they do not allow seat belt extenders in seats that have inflatable seat belt (these are a seat belt that includes a device similar to a car airbag that inflates in the event of a crash).
That same page includes a list of aircraft types that include inflatable seat belts, which appears to include at least some of the the "Big Front Seats" on most aircraft, as well as exit row seats on some.
There do appear to be a few Big Front Seats that do not have such a seat belt, in particular those in row 2 of all aircraft except one model of Airbus A320. Given you have already purchased the BFS, I would suggest simply trying to get a seat in row 2. If you do end up in a seat where an extender is not allowed, and if you do require one, the flight attendants will arrange something for you - either swapping with another passenger in a BFS which does not have an inflatable seat belt, or at worst swapping with someone in a normal seat.