You can disable the Reader feature, which will hide the icon. If you never use it anyway, this is probably the simplest method.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste reader and pause while the list is filtered

(3) Double-click the reader.parse-on-load.enabled preference to switch the value from true to false

That's it, you can close the tab now. If you are currently seeing the reader icon in the address bar of an open tab, if you reload the page, it should not reappear.

Oops, wait, that was for desktop Firefox. Let me look at Android.

That did the trick

Thanks

The same method is available on Android, but its operation is a bit different:

(1) In a new tab, type or paste about:config in the address bar and tap Go. Tap the button accepting the risk if presented.

(2) In the search box above the list, type or paste reader and pause while the list is filtered

(3) Tap the reader.parse-on-load.enabled preference to enable the Toggle button, then tap Toggle to switch the value from true to false

That's it, you can close the tab now. If you are currently seeing the reader icon in the address bar of an open tab, if you reload the page, it should not reappear.