www/mozilla-firefox: compose sequences?

[naddy_(_at_)_mips_(_dot_)_inka_(_dot_)_de (Christian Weisgerber)]

I accidentally noticed that text input fields in Firefox accept
rather more compose key (Multi_key) sequences than the expected
Latin 1 set.  For example, I can input all the characters with
diacritics required for Czech, Slovak, and Serbo-Croation.

Where do those compose sequences come from?
X11, Gtk+2, or Firefox itself?

The available sequences don't match any of the individual sets in
/usr/X11R6/lib/X11/locale/*/Compose.

I'm asking because it would be nice (1) to have a reference table
of all implemented sequences and (2) to be able to modify it, e.g.
l with stroke is missing for Polish.

-- 
Christian "naddy" Weisgerber                          naddy_(_at_)_mips_(_dot_)_inka_(_dot_)_de