If you are logged into Twitter you should see a quill icon to compose a new tweet. But, both in MiniBrowser and GTKLauncher you only see a blue square without the icon.
I'm also seeing this with Chromium 27.0.1425.0 (dev channel). The close icon is also not showing in the top right corner of popups containing profile summary.
Changing component since it seems to be a core bug.
Bug #111241 already has a reduction case on hands and a regression range. Let's close this one as a dupe. *** This bug has been marked as a duplicate of bug 111241 ***