Regardless of the underlying implementation, Filler and Designer serve completely tasks. Designer is a tool for creating InfoPath forms and Filler is a tool for filling them out. Neither is a restricted (or "castrated") mode of the other. They are two completely separate execution modes of the InfoPath executable.
Anyway, is there a point for this thread, or did you just want to complain?
To respond to your comment about list forms, list forms are a special type of form that are directly linked to a SharePoint list. They have features that make some list interactions easy to implement, but they also have a lot of limitations on account of this tightly-coupled, simplified design. They are fundamentally different from schema-based InfoPath forms, and that's why you can't switch between one and the other.
To answer your question in the thread's title, if you're just looking to create browser-compatible forms, all you need to do is open InfoPath Designer and select Blank Form or Blank 2007 Form. Those are browser-compatible by default.