Mozilla/5.0 (Windows NT 6.1; rv:45.0) Gecko/20100101 Thunderbird/45.1.1
In short, the latest git head will give you many bug fixes, a working
PPP, IPv6, beter SNMP, and maybe some yet-to-be-found new bugs.
The UPGRADING file is yet to be updated, and I didn't upgrade myself to
comment on that. As I've been following the posts in the devel list, I
expect differences on the address macros (due to the introduction of
IPv6) but the API should remain the same. PPP will probably be totally
different, and IPv6 is new.