Hi,
I found the issue. I have a second Firefox profile directory, which holds another copy of libwidevinecdm.so, but that copy is (for some reason) truncated to 8_388_608 bytes, whereas the version present on my main Firefox profile is 18_733_688 bytes in size. Comparing both files revealed that the first 8_388_608 bytes are identical. Deleting the libwidevinecdm.so truncated to 8 megabytes solves the problem. Maybe I had a filesystem corruption (using ext4), or maybe Firefox messed up when downloading that version at some point.
Kind regards.