I have the same problem, read all the posts, spent over $6,000 in 'tuneups' over the past 5 years, and on one can find the problem. Not even click and clack.
It started out really small when I bought the car (used, 6 years ago), so slight I thought it was the tires. Fast Forward 6 years: it is now so bad I can't run the heater. Not so bad if I have the re-circulation on. So its coming from within the heating system, or the heating/vent intake is right where the source is. Since it has been going on for so long, there must be a LOT of rubber burning! It smells exactly like burning rubber tires, not oil, fluid, plastic, belts, gaskets... rubber. I can't believe it can't be found, it just doesn't make sense. If I find out what it is, I'll post that.
Oh, btw, I have a 2003 Subaru Baja, not a Forester, but I Googled "subaru burning rubber smell" and it brought me here!
Also, BTW, I do smell it from outside, and from beneath the hood (when I open it), but it is not that strong. It is there, but not nearly as bad as inside the cabin.