Step 1: Open the Options dialog:

 

Step 2: Locate the "FireShot API" checkbox and check it:

 

Step 3: Click "Save and close" to close the Options:

 

Step 4: Confirm permissions:

 

Step 5: Once you've enabled the API, please manually reload all the open tabs.

This is just a one-time action: all new tabs will be enabled for the API automatically.

 

Congratulations: you're done!