Where I work, we have 2 public Ubuntu computers. I want to set up a custom message to be prompted whenever the user opens Firefox.
James is correct, however if you wanted an actual pop-up box:
First, make a new file somewhere called runffox.sh, make sure to create it as your user. e.g. /home/username/runffox.sh
Inside it put
Where MESSAGE is the text you want to show them.
Then in terminal
This allows all to execute the file, but only user to edit it. If you want only root to be able to edit the file change 755 to 555
Now in terminal type
Now, if you launch firefox from the sidebar or desktop etc, it will show the message, then open firefox after you close the message.
Also, for more options with zenity to edit the popup message,
p.s. I'm not sure if a two part command can be put after Exec=, if anyone with more experience could verify, might be able to skip creating the .sh file.