[laurent123456]

Using the Android mobile app, create a Fastmail link and add it to your home screen. Open the app, try to block a sender - it asks you to confirm and it's not possible to press either buttons.

The same workflow works fine in Chrome.

[kiwijamo]

Yep I can confirm that certainty doesn't seem to work in a reliable way in Firefox on Android. I could only get it to work by tapping the button several times. Either it's not responding quickly enough, or the input field only works if you tap in a specific spot. The general performance of Fastmail in Firefox on Android is quite poor as well -- despite it being fairly snappy in desktop Firefox on Linux/Windows. I use the native Fastmail app on Android which performs much better -- I presume it uses a version of Chrome under the hood.