An alternative way to install it would be to download the APK on a computer and then install it via the adb command line tool using a USB connection between the computer and the FP1.
It would be roughly like this
- download the briar mailbox APK from their homepage at https://briarproject.org/apk/mailbox.apk to your computer
- download the SDK Platform Tools release notes | Android Studio | Android Developers that fit the operating system of your computer
- enable the developer options on the FP1 as described in Dictionary Entry: Developer Options
- in the developer options of the FP1 enable “USB debugging” (or maybe it was called “ADB debugging” back then, I don’t remember)
- connect the FP1 to your computer with a USB cable
- type
adb install mailbox.apkinto the command line windows of your computer (assuming the platform tools from the earlier step have been added to the $PATH and you have changed the current working directory to the one where the APK file is stored) and press enter - on the FP1 there should then be a dialog that asks you if you want to allow access via USB; you have to grant the access