The gmpwidevine.sh with the firefox env is currently installed into /usr/lib64/chromium-browser instead of /etc/profile.d. It looks like that exeinto should be an insinto! I made the change locally and that got it working. Thanks for providing all this in the first place :)
https://github.com/chadmed/asahi-overlay/blob/5c34de4b57a089b3701f9461874f62c6fc7a75da/media-libs/widevine-installer/widevine-installer-0.1-r1.ebuild#L36-L37
The gmpwidevine.sh with the firefox env is currently installed into /usr/lib64/chromium-browser instead of /etc/profile.d. It looks like that exeinto should be an insinto! I made the change locally and that got it working. Thanks for providing all this in the first place :)