Firefox Android uses the same button for Stop and Refresh depending on the status of the webpage if it is loading or not. The problem is, both Stop and Refresh are useful regardless of the status.
If the webpage is already loaded, only Refresh button is available. Stop button should also be available to stop whatever actions the page is doing, for example, stop playing videos or stop refreshing scripts.
If the webpage is still loading, only Stop button is available. Refresh button should also be available as an alternative action when a webpage is loading slowly. Refreshing the webpage is a logical attempt.