Must I use the K9 Browser, or can I use the default Android browser (or some other browser)?
Due to the way that the Android operating system is designed, this is not currently possible. The Android operating system uses a concept known as "sandboxing", which prevents apps from accessing or modifying data of other apps. This means that the K9 Browser app cannot "see" information that is accessed by other apps on your device. There has been an issue logged with Google, asking that they provide the appropriate hooks to allow K9 to work for all browsers. You can star and follow that bug report at code.google.com. If Google adds the appropriate hooks for us, we will have more flexibility in our filtering options on Android.