1) Beats me. I've tried to find this out in the past and all I can say
is that it doesn't seem to be either a Firefox or a bash option.

2) You can find a (slightly out of date) list of Firefox command line
options at

https://developer.mozilla.org/En/Command_Line_Options

and a smaller but more up to date list at

http://kb.mozillazine.org/Command_line_arguments

3) Not so far as I know, but I'm sure that if it is possible, someone
on this list will tell us.

See here for confusing reading:
standards.freedesktop.org/.../...-0.9.4.htm\
l#legacy-mixed
About half-way down the page is: List of valid Exe parameter variables

The %u in firefox (or other launchers) seems to be depreciated, but I
think (from what I had read) that it is intended to allow drag and
drop opening of a single file onto the launcher. Coincidentally, this
works currently with or without the %u, which is why I am thinking
that it is depreciated (if that word is right... I don't do so well
with big words!). However, If you replace with %U, then restart
gnome (killall gnome-panel), you can highlight several files and drag
and drop them onto the launcher, and they will all open. If you try
that with the little "u", you will open the first file, and get an
error message on the second. At least that's what my system does.

I think that %u is for a location. It could be a url or file location. See:
https://help.ubuntu.com/7.04/user-guide/C/launchers.html

Regarding a delay on starting firefox (3) There are two ways that
should work. However, I have never had luck with the first. Maybe
someone can clue me in as to why.

1) go to System -> preferences -> startup applications -> add

Then type "sleep 30 && firefox &" into the command box... But maybe
the problem is the launcher is not in a bash shell?

2) What has worked for me:

make a script by opening gedit and typing the following

#!/bin/sh
sleep 30 && firefox &

(note that is a "# "and a "!" without any spaces between or after them
on the first line.)
Save script as something like "firefox.sh"

go in your terminal and navigate to the directory you saved it to,
then type "chmod +x firefox.sh" to make the file executable.

Now to test the script, either type ./firefox.sh, or doubleclick on
the file and select "run". If it is working properly, you should be
able to add the script to startup the same way you added your
application (i.e., System -> Pref -> Startup Apps -> Browse).