When I take my clothes out of my Frigidaire dryre they often smell musty, especially cotton items, what is causing this?

If they are left wet too long before drying, say, more than a 24-36 hours, they can acquire a musty smell that will be more noticeable after drying. This also depends on what soaps you use.

Depending on the dryer however, you may have a mass in your air intake. Could be a mouse nest or just dust bunnies gone bad.

Perhaps it is the liquid softener you are using. It could also be the lint filter. You should clean the filter out often. Also try using fabric sofener sheets. They can give your clothes a fresh scent.