Firstly, to use the correct terminology, the part of the body that they are attached to is the fender not firewall. The firewall is the back of the engine bay that separates the engine bay from the passenger compartment.
Yes the longer cable does not belong there. Depending on whether you have the Duratec (known as 24v or 4v) engine or Vulcan (2v) engine, it grounds somewhere else.
The location is either step 5 or step 6 on the attached. Hopefully someone else can provide better pictures of the location and advise on accessibility but generally speaking the longer cable goes to the left.
Please note that the attached are removal instructions. You need to follow the instructions in reverse to do a correct installation.
For safety, you should disconnect the negative cable entirely from the battery first and put it somewhere where it can't touch the terminal. Then take the longer cable and attach it to the location in step 5 or 6 as appropriate. Then reattach the negative cable to the battery.