I installed Firefox and Thunderbird in /usr/local/
I entered the following commands as per instructions I received, (to allow these applications to be accessable to all users from any directory I think??);

# ln -s ../firefox/firefox .
# ln -s ../thunderbird/thunderbird .

I created 2 GUI launchers and attached the following commands to the respective launchers;
firefox
thunderbird

All worked fine. Any user hit their launcher, and either firefox or thunderbird would start.

Then today I clicked my firefox launcher and received the following error message;
Cannot launch icon
Details: Failed to execute child process "firefox"
(no such file or directory)

The same went for thunderbird.

So I went to the properties of the launchers and changed the commands attached to them from;
firefox
thunderbird
to;
/usr/local/firefox/firefox
/usr/local/thunderbird/thunderbird
Now the launchers work again.

I try to get the launchers working again with the single word commands attached (like they worked before), by trying to create the links again.

I go to the command line and enter the command
[root@myplace myUserName]# ln -s ../firefox/firefox .
I get the response;
ln: './firefox' : cannot overwrite directory

I think maybe I am entering this command from the wrong directory, so I enter it from the next directory up;
[root@myplace home]# ln -s ../firefox/firefox .
I get the response;
ln: './firefox' : File exists

I change directory again to the next one up and enter;
[root@myplace /]# ln -s ../firefox/firefox .
I get the response;
ln: './firefox' : File exists

Why do I now have to attach the entire path to the launcher commands?
Why did the launchers spontaneously stop working with the single word commands attached?
Has something happened to the links?
Can anyone shed some light on what is happening here?

You were originally given bad advice about the use of the "ln" command. The man page for the "ln" command is not very clear either.

If you just want to enter the name of the command to launch the application then you need to have either the real application executable or a link that points to the application executable in a directory contained in the PATH environment variable. If your firefox executable is located at /usr/local/firefox/firefox then do this.
This will create a link named firefox in the current directory. That link will point to /usr/local/bin/firefox/firefox. Since the /bin directory is always in the PATH environment variable then when someone enters the "firefox" command the application launcher will search in /bin and find the "firefox" link that points to the firefox executable in /usr/local/bin/firefox. This is how I remember the syntax of the ln command.

ln [-s] real-file link-name

Thanks a lot for your highly informative response.

I did find it odd how the launchers were working fine as I had them set up, then the next day they did not.