user: Question: What percentage of users were using Firefox according to the source that reported 15.40% used Safari?
Columns: ['Source', 'Chrome', 'Internet Explorer', 'Firefox', 'Safari', 'Opera', 'Other']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['StatCounter', '42.68%', '25.44%', '20.01%', '8.39%', '1.03%', '2.44%']
SQL:

assistant: SELECT "Firefox" FROM "Table" WHERE "Safari" = '15.40%';