I got a solution, I'm not very sure it is root and proper or not though.
Before I ran "slackpkg upgrade-all", Konqueror was default web browser.
I couldn't remove its launcher icon permanently from the panel because it is default web browser.
(Konqueror launcher icon appear again after reboot, although it was previously removed from panel)
But I usually used firefox putting its launcher icon on the panel.
(without setting as default)
This was the situation before I run "slackpkg upgrade-all".
When I ran upgrade, it upgraded firefox from the version 15.something to 20.0.
The point is here,
it seems that firefox took up as default web browser in upgrade process.
But it was possibly in improper way.
So, the system still knows the default web browser is Konqueror.
But, on the panel, the default browser is firefox.
That's why, after reboot, the panel appears with a blank at the place of default browser.
But a launcher is there without icon so that I can click on it to start up firefox.
Ok! my solution is this:
go to System Setting > Default Application > Web Browser and chech on "in the following browser".
and then chose "mozilla-firefox" in the box below.
then click on "apply" button and reboot system.
after reboot process, okkkkk! the panel appears with the firefox icon at the place of default browser.
that's all I did.
My tech knowledge is very poor.
So my conclusion may be wrong and
my solution may not be solving the root of the problem.
thank all for help thinking my problem.
Last edited by mshlinux; 04-24-2013 at 11:00 PM.