Pat steak dry with paper towels. If there is If pieces are too long to fit on grill, cut into manageable sections.
In a gallon-size zip top bag, mix together the olive oil, orange juice, lime juice, vinegar, soy sauce, molasses, garlic powder, onion powder, chili powder, and salt.
Add meat to bag. Squeeze out most of the air from the bag and seal. Arrange bag so all meat is in contact with marinade.
Marinate at cool room temperature for two hours, on in the refrigerator for up to 24 hours, turning the bag over occasionally.
About 45 minutes before you're ready to eat, preheat grill to high.
Remove steak from bag and let excess marinade drip off, leaving a little bit clinging to the meat.
Arrange meat in a single, uncrowded layer on the grill, all over direct heat. Grill the steak until nicely browned on the underside, 3 to 5 minutes.
Flip once and grill until internal temperature reaches 120°-125°F for medium-rare. (Temperature will continue to rise while the meat rests.) Timing will vary quite a bit depending on the cut and thickness of the steak. A thin skirt steak will be done very quickly, and a thicker cut will take longer. While personal preference plays a part, it's widely agreed that all of these cuts thrive at medium-rare.
Let steak rest for 10 minutes, then slice thinly against the grain.
While steak rests, warm refried beans and tortillas and prepare the toppings. Set out ingredients buffet-style or assemble tacos by layering some beans, steak, avocado, onions, cheese, and cilantro onto each tortilla.