I've talked about this problem in the past... the FreeTextBox control embedded on .Text is not compatible with Firefox and so it's impossible to perform a WYSIWYG post by using the blog engine admin interface. The required action is an upgrade to the new FreeTextBox 3.0 control, that support Firefox and all the Gecko-based browsers, but obviously this process requires a .Text compilation and some works on customizations.
After having seen some posts about this integration (not too much clear), today Brendan Tompkins has post on his blog a detailed explanation oh how to integrate the new FreeTextBox 3.0 on .Text.
Really a cool post... today is too late, but I've just scheduled what will be my next Blog customization! 