O newline
para sed
' a
ppend é' quebra ', então você precisa inverter o newline
ou sustentar% símbolo \n
:
sed -i '' '/user_pref("mail.identity.id1.reply_on_top", 1);/ a\
user_pref("mail.identity.id1.sig_file", "/Users/illias.seba/Library/Mail/V2/MailData/Signatures/signature.html");\
user_pref("mail.identity.id1.sig_file-rel", "[ProfD]../../../Mail/V2/MailData/Signatures/signature.html");\
' ~/Library/Thunderbird/Profiles/*.default/prefs.js
ou
sed -i '' '/user_pref("mail.identity.id1.reply_on_top", 1);/ a\
user_pref("mail.identity.id1.sig_file", "/Users/illias.seba/Library/Mail/V2/MailData/Signatures/signature.html");\nuser_pref("mail.identity.id1.sig_file-rel", "[ProfD]../../../Mail/V2/MailData/Signatures/signature.html");\n' ~/Library/Thunderbird/Profiles/*.default/prefs.js