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/tmp/tmp.EB97rs4a6X/z /var/tmp/dist/firefox
/var/tmp/dist/firefox
Starting build: Fri Apr  3 08:07:15 2020
/var/tmp/dist/gcc/bin ~
~
patching file mobile/android/app/src/main/assets/publicsuffixlist
patching file netwerk/dns/effective_tld_names.dat
Hunk #1 FAILED at 5480.
1 out of 1 hunk FAILED -- saving rejects to file netwerk/dns/effective_tld_names.dat.rej

where the namecoin eTLD patch (projects/firefox/namecoin-etld.patch) does:

diff --git a/netwerk/dns/effective_tld_names.dat b/netwerk/dns/effective_tld_names.dat
index 9dd962a..3402b20 100644
--- a/netwerk/dns/effective_tld_names.dat
+++ b/netwerk/dns/effective_tld_names.dat
@@ -5480,6 +5480,7 @@ pro.om

 // onion : https://tools.ietf.org/html/rfc7686
 onion
+bit.onion

 // org : https://en.wikipedia.org/wiki/.org
 org

It's an easy fix.