You can set various preferences for the editors that JBoss Developer Studio adds to the Eclipse environment by selectind JBoss Tools > Web > Editors .
In the initial Editors screen, you can decide on some global settings for JBoss Tools editors. You can select whether an available JBoss Tools editor should always be the default editor for a type of file, whether the user should be warned that making a project an Red Hat project will make an JBoss Tools editor fully available for a particular type of file, and whether, for JBoss Tools editors, the Source mode should be the default instead of a visual mode.