Moreover, when degraded, the rubbery foam turns into a goo-like substance which will stain your focusing screen, mirror, or lens, contaminate the film compartment and even crumble over your film. It is very sticky and nasty, so it gets harder to clean when it gets worse.
But, you can find replacement light seals in bulk from ebay (search: light seal replacement, i.e., this one). Unless they are custom made, they are sold in bulk packs which you can cut/customize for your needs (most sets can fix over 3-4 cameras).
This instructable will show you how to apply them. I will start with my old Canon F-1 (I sold it long ago unfortunately). It was in great shape, except for the rotten foam light seals. They needed to be changed in time, so here is what we'll do next:
1. Identify and document the existing rotten seals,
2. Scrape off the seals, clean the surface,
3. Cut the new seal foam in correct size and shapes,
4. Apply the new seal foams in places.
There is not much risk in this operation, but you can still ruin the focusing screen, the mirror, and the shutter curtains if you are not careful. So be warned, and don't start if you don't feel comfortable.
Step 1: Inspect the existing light seals for condition.
Step 2: Clean the film door area.
Be careful to keep your fingers off the shutter curtains. They are very fragile, you don't want to ruin them.
Step 3: Clean the mirror box area.
Be careful and do not touch on the mirror. It can be stained/damaged very easily.