Your input is encoded double. Using Python 3:
urllib.parse.unquote(urllib.parse.unquote(some_string))
Output:
'FireShot3+(2).png'
now you have the +
left.
Edit:
Using Python 2.7, it would need to be:
urllib.unquote(urllib.unquote('FireShot3%2B%25282%2529.png'))