Hello,
I wanted widivine in kodi, after some fails it found a solution.
Maybe this helps you to get widevine working on arm aarch64 in web browser, too.
First I used widevine-installer (needs squashfs-tools package) from here
https://github.com/AsahiLinux/widevine-installer
This should already work with chromium or firefox, didn't work in my case.
I did not try further to solve web brower issue, cause I wanted to use kodi.
I installed kodi via flatpak and setup widivine with inputstream-helper plugin.
And again it failed and was not working 😞
Final solution was to copy libwidevinecdm.so created by widevine-installer (/var/lib/widevine/libwidevinecdm.so) to kodi cdm folder (/home/user/.var/app/tv.kodi.Kodi/data/cdm/libwidevinecdm.so)
So should be possible to get widevine working in chrome or firefox, too !!!
Good luck !
Bjoern