After installing the latest Firefox 2.0 alpha, I realized my VS 2005 no longer could start debugging. It complained with a “Could not start program …\firefox…” message as it couldn’t find my browser anymore after I upgraded it. After a quick Googling, found a blog with the answer. Right-click your web project, and select Browse With…. Presto! The menu you were looking for…

Dunno if this works in previous versions; I rarely used alternative browsers back then, or had to dink with this setting.