I would always bf the baby before offering solids until she is a year old. Breastmilk has everything your baby needs for its first year of life. Solids are introduced at 6 mo, as a sort of trial for babies to learn to eat from a spoon. She needs to fill up on breastmilk instead of solids. If she is still hungry she will eat the jarred food too, but don't be surprised if she can't finish a whole jar. The more you breastfeed, the more milk you will make, so yes it would help your supply.
Please post if you have anymore questions! Keep up the great work!
