In dialogs, text entry fields (HTML Forms) and any other place where a text entry field might pop up, pressing tab to change the focus to that field doesn't select the contained text. If you tab from the URL field to the Summary field, for example, the text in the summary field should get selected. This behavior is standard and expected.
*** This bug has been marked as a duplicate of 28583 ***
Verified, this is a duplicate of bug 28583 "Tabbing to text widget should select widget contents."