Old Marshall Valvestate Amp's circuit board is covered in a hard foam?

Hi,

I own an old 100W Marshall valvestate amplifier. It hasn't worked in years but recently I opened it up to take a look, in the hope that I could fix it.

However upon taking it apart I noticed that the circuit board was covered in a hard foam-like substance!?

The foam-like substance was very much like 'expanding foam' (i.e., the stuff builders use to fill gaps in walls etc). It was a brownish colour, porous and hard to the touch, but it was easily breakable (I could chip away at it using my fingernails)

There looks to be a fairly large capacitor on the circuit board, from which it appears that the foam has emanated from, but I cant be sure as the foam has literally covered all the components on the board!

Furthermore a blown capacitor wouldnt spew a foam that hardens??!!

1) What's going on with this circuit board?

2) Any suggestions as to what this foam is or how it got into the amp? (Is it put there intentionally as a heat sink or something crazy?)

Also the amp used to work but over time it started to cut out until one day it stopped working completely!