OS 10.5 - Need Developer/IT input

Hi. I'm a proficient mac user and I'm still needing a developer/IT guy's help.

I am using OSX 10.5.7 Leopord, 8-core machine, w/newest update of Firefox

Here's my problem...

My mac is for some reason blocking certain ports (or something). Here are two situations to explain...

1. When I'm trying to access cPanel "Example.com:2082" it gives me a "Page Load Error" page saying "Failed to Connect - Firefox can't establish a connection to the server" ... but when I try to connect on my PC, using internet on the same browser & router & ISP, it WORKS. So! Something wrong w/my mac. What I'm looking for is the "User" and "Pass" prompts to login to cPanel.

I've tried on Firefox and Safari on my mac, both fail.

2. Have been unsuccessful using default email port 25, and my ISP has reconfirmed many times they are not blocking it. So, again, my Mac is the issue. I've been forced to buy a dedicated IP "2525" so it works. Not sure why, but that's what had to happen.

Any thoughts/ideas that may be the cause of this, and I can fix??

I've spent a lot of time researching, w/no success.

Thank you ahead of time.