First tap where you want the selection to start. (It might help to zoom in as far as possible first. You can do this using the volume buttons in the newly-released Firefox 1.1rc1.) Then hold the Shift key while pressing the arrow keys on the N900 keyboard to select text.

Note: We are aware that this is not a very user-friendly way to select text, and we hope to include a better design in a future version of mobile Firefox.