Currently on the Network -> Headers tab, you can see the form data
submitted on a POST request. However, unlike with Request Headers and
Response Headers, there isn't a "view source" option to see this form data
in the raw.
This option would be helpful, as it's (a) easier to see subtle syntax
issues this way.
I ran into this problem today, where a piece of form data had quotes around
it when it shouldn't have. Since you often see quotes around string data to
distinguish them from other datatypes (e.g., JSON), this didn't immediately
stand out. I spent a few hours troubleshooting, thinking the problem was
somewhere else, until I used Firebug with Firefox, and viewing the raw data
immediately made the quotes stand out.