Having stumbled my way this far, setting up dovecot pigeonhole and managesieve, and finding no usable extensions for Thunderbird, moved on to configure Roundcube, where I can now find "Create Filter" and click on it, without visible complaint.
However, nothing seems to happen.
Dovecot and access via TB and Roundcube worked fine before and still appear to work, just, not for managesieve.
Kinda/sorta looks like port 4190 not "bound" to anything. Attempt to telnet to localhost 4190 gives connection refused.
Oh, dear, just clicked with me that in the selection box, only "Import" seems active, everything else is "dimmed".
Come to remember, that was the case before today, but "create filter" was not there.
Guidance?