This causes Firefox to always display some blank space before the first line of text, while IE does not.

For the long story, and fix options read the DotNetNuke Forum post at:
http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/127/threadid/153307/scope/posts/Default.aspx

Suggested fixes:

1) In the fckconfig.js file replace (in \Providers\HtmlEditorProviders\Fck\FCKeditor)
FCKConfig.EnterMode = 'p' ; with
FCKConfig.EnterMode = 'br' ;   /* or 'div' */

2) Use CSS to set the <P> properties to:
p {margin-top: 0px; margin-bottom: 3px;}

Both solutions have favorable arguments: The first avoids FCK from modifying the HTML, while the second provides more standardized HTML and a uniform way of controlling the <P> tag formating. Also, check for updates to the DNN FCK Editor Provider, since these issues are currently being addressed.

OK, I've had better results with the CSS solution. Setting the margin-top to 0 solves the problem with Firefox, but I'd rather not set the margin-bottom (use the default, which follows the font size).
So you can set  p { margin-top: 0px; } either in the \Portals\_default\default.css  or in the appropriate  skin.css,  if you don't want to change the default for all portals.