How much is coming out? If it is not much then don't worry about it. This is common on older Benz and I have experienced myself in my car. It is exactly what you thought. Foam rubber. I don't know how big a job it is to replace the old seals, but I would think it is probably not worth it unless you are getting a ton of the foam.

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired