I think normally that is where the lock icon appears indicating whether the page is secure. Could you reload the page bypassing the cache in case the cache is a factor? You can use Command+Shift+R or you can Shift+click the Reload button.

You can check the Security tab in the Network Monitor.

• https://firefox-source-docs.mozilla.org/devtools-user/network_monitor/

Thank you for the good answer. But rather than solving this problem, I'm curious as to why I see a paper icon.

Only the Browser Toolbox can probably show under what conditions you see this icon as I don't know what to look for.

• https://firefox-source-docs.mozilla.org/devtools-user/browser_toolbox/