On my samsung nexus (maguro), running Replicant 6.0, installing Briar via F-Droid fails with the following message:

Error installing Briar Beta
Failed to install due to an unknown error

I pulled the following from logcat:

E Installer: Permissions in APK and index.xml do not match!
E Installer: org.fdroid.fdroid.installer.ApkVerifier$ApkPermissionUnequalException: Permissions in APK and index.xml do not match!
E Installer:     at org.fdroid.fdroid.installer.ApkVerifier.verifyApk(ApkVerifier.java:85)
E Installer:     at org.fdroid.fdroid.installer.Installer.installPackage(Installer.java:234)
E Installer:     at org.fdroid.fdroid.installer.InstallerService.onHandleIntent(InstallerService.java:77)
E Installer:     at android.app.IntentService$ServiceHandler.handleMessage(IntentService.java:66)
E Installer:     at android.os.Handler.dispatchMessage(Handler.java:102)
E Installer:     at android.os.Looper.loop(Looper.java:148)
E Installer:     at android.os.HandlerThread.run(HandlerThread.java:61)
E Installer: Falling back to AOSP DefaultInstaller!

This issue seems to be tracked in https://gitlab.com/fdroid/fdroidclient/issues/1139.