What causes my saliva to be foamy in my mouth? Does that mean it is acidic?

No. Saliva is always foamy because it contains some mucus to make it slippery. Generally, ignore stuff about "acid" and "alkaline" at pop websites; it's often totally made up. The foam produces bubbles which get swallowed, and this accounts for much of ordinary intestinal gas, and why talking a lot and chewing gum makes people pass more of it.