<head> — still no Typekit fonts.
Turns out the culprit was the disable referrers setting in the Web Developer Toolbar extension for Firefox. Under the Disable tab there is an option to disable referrers — if it is checked it will not allow any Typekit fonts to load properly.
You may also want to check your referrers settings in about:config. To do this open a new tab in Firefox and type about:config in the URL bar and hit enter. In the filter box type network.http.sendRefererHeader, and make sure the value of the network.http.sendRefererHeader preference is set to 2 (not 0 or 1). To change the value, double click on the preference row.
Thanks to Sean at Typekit for helping me get this solved.