Guys I'm posting up an answer here since it came up in a search and I didn't know where else to put it... 10 years late LOL.
Today I found a cheap and readily available replacement part for the fragile old style Stihl Foresty Helmet System cap-mounted earmuff clips.
Mine have broken twice, and most recently I could not find a stihl replacement at a decent price.
Stihl's part number is 0000-886-0406, good luck.
But... I noticed that Tasco has several models that are much cheaper and come with identical hinges can clips:
Search "tasco cap-mounted ear muff" and you will see a "golden eagle 2951" and "sound star 2551", to name just a few.
(it appears that the model 2551 in black only lacks the stihl logo, but is otherwise the same muff as oem for ~half price)
These are IDENTICAL from the hinge to the cap, with only the muff varying in size shape color logo or noise level etc.
The auction site I visited yesterday had 1 new old stock stihl earmuff set for sale at 39.99, where the golden eagles were $20.31 new with prime, sound stars were $29 ish.
It's very easy to either replace the stihl muffs with these new ones, or you can just remove the plastic mounts, take them apart with a phillips screwdriver, and put back together using your old stihl ear muffs.
I received mine today and can confirm, the clip and hinge is identical side by side, you can't tell them apart.
Cheers!