Ubuntu Touch: no onscreen keyboard in Firefox?
Searching some more I found that this seems to be a known bug actually: https://bugs.launchpad.net/canonical-pocket-desktop/+bug/1543351
This is the latest comment at this point:
kevin gunn (kgunn72) wrote on 2016-04-15: #6
just confirming, this is definitely on our todo list as a high priority.
UPDATE 2016-07-06: the fix is currently in testing, and has been available through the rc-proposed channel for the last week. It is expected to be available in the OTA-12 stable release (mid July).
UPDATE 2016-12-27: the fix is out in stable for some time now.