Solved it for me by editing /etc/apparmor.d/usr.bin.firefox on XSE (Debian- & Xubutnu 18.04 LTS-based distro):
# per-user firefox configuration #... owner @{HOME}/.{firefox,mozilla}/**/gmp-widevinecdm/*/lib*so m,
That solved the problem when using Mz Firefox 100.0 (64-bit) and widevine 4.10.2449.0
Solved it for me by editing /etc/apparmor. d/usr.bin. firefox on XSE (Debian- & Xubutnu 18.04 LTS-based distro):
# per-user firefox configuration /.{firefox, mozilla} /**/gmp- widevinecdm/ */lib*so m,
#...
owner @{HOME}
That solved the problem when using Mz Firefox 100.0 (64-bit) and widevine 4.10.2449.0