+1 what TripleR and others have said. The BX tractor itself is quite stable, as tractors go, because the comparatively small tires keep everything close to the ground. But when you install the FEL and carry something in it, you shift a large amount of weight to the front, where the front axle is on a pivot, and that makes it very tippy. Raising the FEL higher amplifies the problem, of course. The solution is putting as much offsetting weight as possible on the back of the tractor, so the back axle, which does not pivot, stays on the ground. Ballast in the tires, a heavy implement on the 3 point hitch, an operator who wears XXL garments, you name it. And using wheel spacers would also help.
Even if you get it all set up the right way, you still have to use good judgement when working on uneven ground. But if you are careful, you can do a lot of work in places other equipment can't get into. Good luck!