FWIW: these are "Rapidfire" shifters, where the shifting is activated by clicking on a lever or trigger under the handlebar. What is normally called a "thumbshifter" is a single lever mounted above the handlebar. It's a piece of bicycle mechanic trivia that might help, particularly with surly old farts like me.

Now, that SunTour XC Pro MD derailleur was never designed with index shifting in mind. The MD means "Microdrive" so the cage is shaped to fit a small (42-tooth) front chainring, so that might work in your favor. The sticky point is the cable-pull-to-cage-movement ratio. This affects everything about the shift and I frankly don't know if it will work. Don't get me wrong... if it were my bike I'd try it out and see what happened. I'd set it up so the cage was centered over the chain when the shifter was in the middle click. If it made the shifts to the big and small chainrings from there, it might work. Test rides and tweaks would certainly be needed- front derailleur adjustment takes a lot of patience and a chunk of understanding what's supposed to happen versus what's really happening.

Your original front derailleur was a Shimano Alivio (the big "A"). It's a sturdy, reliable unit. I'm surprised it didn't fit the chainring... the cage should fit a 42 or 44 tooth chainring well.