What is contextual launching?
Contextual launching is the new, simpler way of launching in Eclipse. It is the default in 3.3 M7 builds and later. Instead of launching the last launched configuration, pressing "Run" on the toolbar or run menu will launch the current selection. For example, if you have a Java file open in the editor, pressing "run" will launch it as a Java application.
How do I turn off contextual launching?
If you just can't get over the old way of doing things, you can do so on the Run/Debug > Launching Preference Page. Change the Launch Operation option to "Always launch the previously launched application".