Many thanks for your comments/suggestions. The firewall pad I used is the one offered (about a year ago) by a group of members of this site, not a commercial vendor. Unfortunately, it is not easy to post pictures, but I can try to describe the problem in detail.
The pad consists of two layers, a top thin layer (with the correct diamond texture) and a thicker felt layer. These layers were delivered pre-cut to the (correct) exact same shape, but completely separated. One surface of the felt layer had already been coated with glue and covered with paper. According to the installation instructions, this paper cover must be peeled off and then the two layers brought together to form one piece that, in turn, is to be glued on the firewall.
The problem (which developed gradually over the past year) is that, while the felt pad is still glued on the firewall, the top layer, for some reason, has badly wrinkled and separated from the felt pad in several places. This suggests that the two layers were not properly bonded together before the pad was glued on the firewall, but am not sure if this is really the cause of the problem. At any rate, my replacement pad looks terrible, certainly much worse than the original pad, which was in passable condition.
I know that several members of this site bought this same pad (whose appearance, by the way, is very close to that of the no-longer-available original), and I wonder if anybody else has had a similar experience. Many thanks.
Lakis
PS. Kevin, I live in Newton and work in Cambridge.