The new private browsing window message has confusing text on Seamonkey 2.35 when using a custom theme, if the theme does not include aboutPrivateBrowsing.css. I have verified this bug on Windows 2003 x86 and Windows 7 x64.
Theme Developers: Include aboutPrivateBrowsing.css into the chrome/communicator/ directory of your theme.
aboutPrivateBrowsing.css: http://mxr.mozilla.org/comm-central/source/suite/themes/classic/communicator/aboutPrivateBrowsing.css
Please include or update aboutPrivateBrowsing.css in your SeaMonkey theme.
example: http://mxr.mozilla.org/comm-central/source/suite/themes/classic/communicator/aboutPrivateBrowsing.css?rev=c9452fadc613
Phil