O.K. since the setting are so important in order to get this working correctly, I will give you the settings that I am using that are working for me, in both IE & FireFox.
Code:
<div align="center"><img src="http://www.YourForum.com/forums/images/rated_nc-17.jpg" border="0"><p><b>This Section Of Our Forum Is Intended For Mature Audiences Only!<br>As such it may contain material that is neither suitable nor appropriate for those minor age,<br>Sexually-oriented adult content which may include visual images and verbal descriptions of nude adults. Adults engaging in sexual acts, and other audio and visual materials of a sexually-explicit nature.</b></p><p><b><font weight="bold" color="#ff0000">Permission To Enter This Forum Section And To View Or Download Its Contents Is Strictly Limited Only To Consenting Adults Who Affirm That The Following Conditions Apply:</font></b></p><p><b>That you are at least 18 years of age or older, and that you are voluntarily choosing to view and access such sexually-explicit images and content for your own personal use.</b></p><p><b>That you intend to view the sexually-explicit material in the privacy of your home, <br>or in a place where there are no other persons viewing this material who are either minors, <br>or who may be offended by viewing such material.</b></p><p><b>If all of these conditions apply to you, you are given permission to enter and continue forward.<br>But if not, you don't deserve any permission to enter and view all the contents of this forum section and you should now exit or return to previous page. </b></p></div>
Splash Warning Height & Width = 500x800
Splash Warning Position = 100,100
All other setting where left with the default ones.
Hope this helps.