No UI mod does anything to the launchpad - the UI is not loaded or accessed whatsoever by the game until the actual client process (everquest2.exe) gets launched from the launchpad. Custom UIs get used as a patsy by CS - when it comes to things like that just don't bring them up or answer "No" if they ask about them.
As for why it wouldn't go through - the fact you can access it in Firefox doesn't mean it's not firewalled, it's a different process entirely. The easiest way to confirm is to simply turn your firewall off (briefly!) and see if it loads. If it still doesn't load, then it's not the firewall at issue.
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