This one is very simple, but not obvious as first...
I tried [% user.firstname FILTER lower,ucfirst %] but my output was lower only. The solution:
[% user.firstname FILTER lower FILTER ucfirst %].
There is also a short hand version:
[% user.firstname | lower | ucfirst %].
Note that filters are processed left to right so [% user.firstname FILTER ucfirst FILTER lower %] would produce only lowercase output.
