Earthlink webmail -- logging on

I normally use a Mac PowerBook G4 running the latest version of OS X Tiger (I get the upgrades all the time). I can log into Earthlink webmail and go away from the page. When I come back I’m still logged in. I can open another window, go to the page, and I’m logged in in the new window. At the office I use a Dell PC laptop. There I have to log in every time I go away from the page and come back. On my PC desktop I stay logged in in the same window, but have to log in again if I open a new window. Right now I’m on my iMac G3 running OS X 10.3.9 (and I’m also on a dial-up, since I see no reason to pay for cable at two places) and I have to log in every time I return to webmail.

Is this a function of the OS? I understand why it would be the case with the PCs vs. the Macs, but I’m curious about the two computers running OS X. Or is there something I’d need to turn on to make the iMac function like the PowerBook when it comes to the webmail page?