I am regular and long time user of Fastmail in Firefox and I'm surprised at your report of issues in Firefox. Can you describe the steps to reproduce the issues you found?
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.
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.
