Hi,
there is completion file _mozilla for mozilla and firefox.
Unfortunately, that completion is not registered for firefox by default.

print ${_comps[firefox]}
_webbrowser

That's because there is a completion Unix/_webbrowser with 

#compdef [...] firefox [...]
that register _webbrowser for firefox
when it comes to X/_mozilla 
#compdef [...] firefox
that function cannot be registered for firefox because compdef is called 
with '-n'
compinit line 478: compdef -na "${_i_name}" "${_i_line[@]}"

So may be, firefox should be removed from _webbrowser

arno


? _webbrowser.patch
Index: Completion/Unix/Command/_webbrowser
===================================================================
RCS file: /cvsroot/zsh/zsh/Completion/Unix/Command/_webbrowser,v
retrieving revision 1.7
diff -r1.7 _webbrowser
1c1
< #compdef amaya arena chimera dillo express firefox galeon grail gzilla hotjava konqueror light mmm Mosaic netrik opera phoenix retawq skipstone www xmosaic zen
---
> #compdef amaya arena chimera dillo express galeon grail gzilla hotjava konqueror light mmm Mosaic netrik opera phoenix retawq skipstone www xmosaic zen