Upholstery foam...is for sitting.
While upholstery foam is great for sofas, chairs and headboards, it's my opinion that it should NEVER be used for mascots and puppets.
The main issue I have with using this foam for building mascots and puppets is that it's an opened cell foam product. Opened cell basically means that it easily absorbs moisture, for example from breathing or perspiration. That can become problematic and unpleasant, especially if more than one person has to wear the costume or use the puppet. In addition, trapped moisture is a perfect breeding ground for all sorts of fungus and bacteria.
Closed cell foam is really the only way to go when making the base for a mascot head in my opinion. E.V.A. foam (Ethylene Vinyl Acetate) is a non-toxic closed cell foam. What that really means to the mascot builder (and performer) is that it won't absorb water and you can easily wipe it clean! Once you have the base shape of your head built, you can block up areas on the outside of the head using either ESTHER foam or ETHA foam to carve details and features.
For puppet building, RETICULATED foam is the best! The main reason is that the pores are so big which allows for flexibility and breathability. Flexibility is obvious but breathability is what I really like. This means your hand isn't going to get as sweaty as it would using upholstery foam and when your puppet is ultimately due for a cleaning you can easily wash it and the foam will dry out completely.
All of the foams mentioned here can also be glued together using Foamfast 74 which is such a great non-misting glue......and let me tell you, (as a guy with hairy arms) non misting is a dream come true. :)
Now get building!