The first time I ran firefox on debug mode, I noticed that when I double clicked on the Google textbox on the browser’s first page, the message:

WARNING: 1 sort operation has occurred for the SQL statement ‘0x7f7f6e10d588′. See https://developer.mozilla.org/En/Storage/Warnings details: file /home/diogogmt/workspace/github-projects/mozilla-central/storage/src/mozStoragePrivateHelpers.cpp, line 139

appeared on the terminal.

First, I went to https://developer.mozilla.org/En/Storage/Warnings to check the details of the warning. It turns out that the warning message tells how many sort operations occurs in a SQL statement, it happens when an SQL query has an ORDER BY statement but does not use an INDEX.

After reading about the warning, I went to the mozStoragePrivateHelpers.cpp file to check which line was displaying the warning on the terminal.
This is the function where the warning is generated.

   
 113 void  
 114 checkAndLogStatementPerformance(sqlite3_stmt *aStatement)  
 115 {  
 116 // Check to see if the query performed sorting operations or not. If it  
 117 // did, it may need to be optimized!  
 118 int count = ::sqlite3_stmt_status(aStatement, SQLITE_STMTSTATUS_SORT, 1);  
 119 if (count 120 return;  
 121  
 122 const char *sql = ::sqlite3_sql(aStatement);  
 123  
 124 // Check to see if this is marked to not warn  
 125 if (::strstr(sql, "/* do not warn (bug "))  
 126 return;  
 127  
 128 nsCAutoString message;  
 129 message.AppendInt(count);  
 130 if (count == 1)  
 131 message.Append(" sort operation has ");  
 132 else  
 133 message.Append(" sort operations have ");  
 134 message.Append("occurred for the SQL statement ‘");  
 135 nsPrintfCString address("0x%p", aStatement);  
 136 message.Append(address);  
 137 message.Append("’. Seeeeee https://developer.mozilla.org/En/Storage/Warnings "  
 138 "details. DEBUG!!!!! My first printf!");  
 139 NS_WARNING(message.get());  
 140 }  

On line 125, it checks if the warning should be suppressed.
On line 128, a nsCAutoString object is created
Some text is appended to the object, and on line 139 the warning is issued.

On line 137, I added some extra text just to see if it would be displayed on the terminal… and it did :)

*I tried to do an incremental build, since it takes more than an hour to build firefox on my laptop, but when I made the changes to the mozStoragePrivateHelpers.cpp file, and re-build only the storage component, it would still display the content of the old file. I had to build the whole project to get the changes working.
MDN Increment build