OpenGL is not 3D only: it does 2D too, ever since OpenGL 1.0 as defined by Silicon Graphics years ago. However, some special memory management features only available in some DirectX 9 and all DirectX10+ graphics card are needed to perform some operations with proper performance - thus, why it took ages for Microsoft to create Direct2D (that has nothing to do with the outdated DirectDraw), and the need for at least OpenGL2.0 hardware.
And proper drivers.
Why no OpenGL support in Firefox for Windows? Answer: not all Windows have OpenGL drivers available...
Moreover, it's not exactly Firefox that added support for Direct2D: to be more specific, Mozilla worked heavily on the Cairo rendering library to add a Direct2D back-end. They are, also, working on adding OpenGL support to Cairo (added in the brand new Cairo 1.10), so maybe for a future revision of Firefox 4, we'll get OpenGL acceleration too...
Note that Cairo is also used by Webkit, so hardware acceleration may also appear soonish in Google Chrome and Apple Safari...