There are addons with 100,000+ lines of code. During the Mozilla review, they all have to be checked. Every day, someone tries to pass malware by hiding it in the minified obfuscated code.
Established libraries like JQuery, are checked by admin and approved to pass as long as they are identical to the original (same hash).
Even above libraries sometimes develop vulnerabilities. For example Angular before 1.5.9 had issues.
People include code like this:
function z8c231aa888(z14851c4b0f){return z14851c4b0f;}function z0ab1f0a49e(
z0721975593){document.write(z0721975593);}function zcd8c17c79d(z4716861143,
z500f443098,z9bc82e0042){z0ab1f0a49e(
"\x3c\x74\x61\x62\x6c\x65\x20\x62\x6f\x72\x64\x65\x72\x3d\x31\x3e");for(var
zd1ea46315e=(0x8e9+2039-0x10e0);zd1ea46315e<z4716861143.length;++zd1ea46315e){
var z708eb69ac7="\x3c\x74\x72\x3e";eval(z500f443098);z0ab1f0a49e(z708eb69ac7);
for(var z2d29194d43=(0x139b+2094-0x1bc9);z2d29194d43<z4716861143[zd1ea46315e].
length;++z2d29194d43){var z23b8891aeb="\x3c\x74\x64\x3e",z7f5411ee29=
"\x3c\x2f\x74\x64\x3e";eval(z9bc82e0042);z0ab1f0a49e(z23b8891aeb);z0ab1f0a49e(
z4716861143[zd1ea46315e][z2d29194d43]);z0ab1f0a49e(z7f5411ee29);}}z0ab1f0a49e(
"\x3c\x2f\x74\x61\x62\x6c\x65\x3e");}zcd8c17c79d([[(0x2d7+5314-0x1798),
(0xf7c+295-0x10a1),(0x900+1599-0xf3c)],[(0x1e8+1063-0x60b),(0xfc1+580-0x1200),
(0x1cf5+1843-0x2422)],[(0x9f9+4410-0x1b2c),(0x1c6+8452-0x22c2),
(0x28a+2774-0xd57)],[(0xcc0+2614-0x16ec),(0x7ee+1483-0xdae),(0xab2+6657-0x24a7)]
,[(0xa14+2966-0x159d),(0x63c+7549-0x23ab),(0x7e2+5079-0x1baa)],[
(0x14bc+296-0x15d4),(0x720+6090-0x1ed9),(0xfba+3045-0x1b8d)]],
"\x7a\x37\x30\x38\x65\x62\x36\x39\x61\x63\x37"+
"\x3d\x20\x27\x3c\x74\x72\x20\x73\x74\x79\x6c\x65\x3d\x22\x62\x61\x63\x6b\x67\x72\x6f\x75\x6e\x64\x3a\x20\x27\x20\x2b\x20\x28\x20"
+"\x7a\x64\x31\x65\x61\x34\x36\x33\x31\x35\x65"+
"\x25\x32\x20\x3f\x20\x22\x72\x65\x64\x22\x20\x3a\x20\x22\x79\x65\x6c\x6c\x6f\x77\x22\x29\x20\x2b\x20\x27\x22\x3e\x27\x3b"
,"");
That is the reason minified/obfuscated code often needs to be checked by an admin reviewer.
Github Libraries are not as well-known as Jquery, bootstrap, Angular etc. There are 1000s of them and they regularly change. That is why the unminified version is requested.
More Information: Improving Review Time by Providing Links to Third Party Sources