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 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1777070 Title: firefox plugin libwidevinecdm.so crashes due to apparmor denial To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/apparmor/+bug/1777070/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs