Why will Firefox not allow me to add swag button to my browser?

How did you try to do so?

https://help.swagbucks.com/hc/en-us/sections/201043124-SwagButton

This problem still exists as of 10/29/2026. The message received is that Firefox is preventing installation of the Swagbutton from the Swagbucks website.

Because they coded it wrong.

You have to dismiss the error, and manually go to the address bar, and hit Enter again, to initiate the install: