I don't believe it is a free text box problem or maybe it is that the new version introduced a problem for DNN 3.3.3/4.3.3.
My solution to the problem was to edit the SetDesignModeCss() Sub in the FtbHtmlEditorProvider.vb file in the Provider.Ftb3HtmlEditorProvider Project. Starting on line 574 I added a new line to create a link to an optional ftb.css file in the skin folder:
'Add FTB Design Mode Css
DesignModeCss += "\' type=\'text/css\' /><link rel=\'stylesheet\' href=\'" + PortalSettings.ActiveTab.SkinPath & "ftb.css"
Then I added a ftb.css file in my skin folder and added styling to the body tag to override the body tag styling in the portal.css file. My ftb.css file looks like:
/* CSS Document */
body {
background-color: white;
font-family: Verdana, Arial, Helvetica, sans-serif;
font-size:80%;
line-height:110%;
}
So it seems to me that the default FTB Design Mode CSS behavior is being overridden by DNN to apply the default.css, skin.css, skinname.css, and portal.css in that order. My change adds a new ftb.css after the portal.css so that you can directly control the FTB design Mode style.