I'm having a similar issue. I also use Firefox as my web browser, and when I went to log in to my account, the Zam network accepts my information, but when I click on Everquest I see ad banners and I get the message that I need a free account to post a message. I also use Windows 7 as well, is Firefox not supported?
Edit: I did what Lyssabeth suggested, and sure enough, logging in to post worked...weird. Thanks for the tip Lyssabeth!
Edited, Jan 22nd 2015 9:50pm by Volonoth