Symptoms
Form viewer web part displaying "Your browser's compatibility mode is not supported" in SP2010 for only Internet Explorer.
Diagnoses
SharePoint's 2010 master page was forcing IE8 compatibility mode via a meta tag found within <head>: <meta http-equiv="X-UA-Compatible" content="IE=8">.
Resolution
Changed this tag to target IE=11 instead, and the form loaded without error, but as this was a global change it affected the functionality of some ribbon controls on a separate site. It is recommended to make this change for the specific site that hosts the Form Viewer.