Are you sure its the thumbie and not the spring in the front mech?
I had an M739 front mech and XC Pro combination on my Mongoose & the front mech kept shifting on its own.
I found that you can adjust the spring tension on the mech, did this and it worked fine. There is a flat blade slot on the mech pivot which has two positions.You can adjust it on the bike with a stubbie screwdriver.
I read about it in the old MBUK Mountain Bike Maintenance & repair book which i have never binned since i bought it Bitd