Very strange, I use pfSense 2.4.5-p1, 2.5.1-REL, 2.5.2-RC, 2.6.0-DEV with FRR doing BGP and this issue has always been solvable.
Only showstopper I have come across these past few weeks is https://redmine.pfsense.org/issues/11545 - If you assign virtual IPv6 addresses to WAN, FRR in pfSense may decide to use one of them for outbound communication, even to your BGP peers which will usually result in the session stuck in "active" and not getting "established" because the peer expects you to come from a specific address.