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.
Created attachment 168370 [details]
Created attachment 168371 [details]
Attachment 168371 [details] did not pass style-queue:
Failed to run "['Tools/Scripts/check-webkit-style', '--diff-files', u'Source/WebCore/ChangeLog', u'Source/WebCor..." exit_code: 1
Source/WebCore/ChangeLog:25: Need whitespace between colon and description [changelog/filechangedescriptionwhitespace] 
Total errors found: 1 in 4 files
If any of these errors are false positives, please file a bug against check-webkit-style.
Comment on attachment 168371 [details]
Clearing flags on attachment: 168371
Committed r131161: <http://trac.webkit.org/changeset/131161>
All reviewed patches have been landed. Closing bug.