>> map ng0 192.168.1.0/24 -> 0.0.0.0/32 proxy port ftp ftp/tcp
>> map ng0 192.168.1.0/24 -> 0.0.0.0/32 mssclamp 1452

> just to double confirm, adding those two rules fixes the problem for you ?

yes

not adding but replacing the ones monowall creates.
Probably "mssclamp 1452" should be added also to the first rule and every other rule
created.

I've noticed that you have removed the option to enable/disable
the "portmap auto" when creating custom outbound rule.
  Is it possible to have it back?