Invoking Netscape Communicator from the command line is
usually done by the script /usr/bin/netscape. This script
does not do "the right thing" in the following case:

    /usr/bin/netscape +composer SomeFileURL

The "right thing" IMHO would be to open a Composer window on
the specified file (starting up Communicator unless there
isn't one running already).

What curently happens is that you get a Navigator window
onto the specifed file. I have tested this for Communicator
versions 4.73 and 4.74.

BTW, the simpler case where you provide the +composer
option but no file URL is handled properly--the Composer
is then opened.

I have found out how to hack the /usr/bin/netscape script so
as to handle the composer-with-file-URL. The maintainer of
the script ought to review this and perhaps come up with a
more general solution.

Here is the output of
diff -e netscape netscape-myProvisionalFix (executed in
/usr/bin, assuming NC 4.74 installed).

---------------- cut
85a
elif [ "$b_opt" = openEditor ]; then
exec $netscape $defs $cl_opt "$@"
.
76a
elif [ "$b_opt" = openEditor ]; then
$netscape $defsrem "xfeDoCommand($b_opt,$url)" 2>/dev/null || \
{ killstale && exec $netscape $defs $cl_opt "$@" ; }
.
---------------- cut