grep(1) -IrL 'p11-kit-trust.so' ~/.mozilla/firefox/*/pkcs11.txt
print lines matching a pattern
-I     Process  a  binary  file  as  if  it  did  not  contain  matching  data; this is equivalent to the
       --binary-files=without-match option.
-R, -r, --recursive
       Read all files under each directory, recursively; this is equivalent to the -d recurse option.
-L, --files-without-match
       Suppress  normal  output;  instead  print  the  name of each input file from which no output would
       normally have been printed.  The scanning will stop on the first match.
grep searches the named input FILEs (or standard input if no files are named, or if a single hyphen-minus
(-) is given as file name) for lines containing a match to the given PATTERN.  By  default,  grep  prints
the matching lines.
source manpages: grep