Unable to explore SP2013 (on O365) apis

Having just upgraded to SP2013 on O365 (Enterprise) I am keen to take advantage of the various REST endpoints to perform various CRUD operations.

However, I seem unable to drill down into the various api properties and values through any browser.

E.g. I am expecting that by entering something like 'https://my-account.sharepoint.com/_api/web' in the browser I should be returned with information about the site.

However, in IE9 I am returned with an 'Internet Explorer cannot display this feed' message (Feed reading view is enabled, BTW).

In FF and Chrome, I am getting a error code : <m:code>-2147024891, System.UnauthorizedAccessException</m:code>
<m:message xml:lang="en-US">
Access denied. You do not have permission to perform this action or access this resource.
</m:message>.

I am an O365 Admin and have site collection admin privileges so am currently at a loss as to why I am unable to access this data.

Can anyone give me some pointers?

Thanks