I have a couple of users experiencing this problem. If the font size in OWA is set for example to 10, when they reply to an email it initially does not assign a font to the first paragraph. The default font appears in the font toolbar when they begin typing the second paragraph. So when the recipient receives the message, the first paragraph font doesn't match the font in the other paragraphs.
Their default IE 8 font is Times New Roman and they cannot set the font size, while their default OWA font is Tahoma size 10.
The only information I have been able to find about this is in this link from Microsoft. However, since they already had different fonts in IE and OWA, it doesn't apply to us.