Testing shows that GW on mobile data is often different after disabling and enabling the data. The changes in bug 1585920 caused that when using mobile data we hash link name as well as IP of GW at https://searchfox.org/mozilla-central/rev/d7537a9cd2974efa45141d974e5fc7c727f1a78f/netwerk/system/netlink/NetlinkService.cpp#1623. That code should cover VPN tunnels so we should avoid hashing linkname+IP when we used this interface at https://searchfox.org/mozilla-central/rev/d7537a9cd2974efa45141d974e5fc7c727f1a78f/netwerk/system/netlink/NetlinkService.cpp#1521.

For same reason, there are artificial MAC addresses available on some non-ethernet link devices. With this patch we ignore all neighbor messages for non-ethernet links. Also, when traffic for the checked host is routed via rmnet_data device, we skip hashing IP of next hop, because it changes very often in case of some mobile providers.

https://hg.mozilla.org/mozilla-central/rev/afba5a084fe4

Does this need a Beta uplift request or can this ride Fx72 to release?

This probably doesn't need to be uplifted. Network change is detected and having different ID for the same network isn't critical.

Hi, verified as fixed with Google Pixel 3a (Android 9) on Fenix/Firefox Preview Nightly 191029 (Build #13020608) and Gecko View 72.0a1-20191025095546.
I will mark the ticket as Verified.