Are you tilting it to the right(side buttons on bottom)? Try tilting it to the left. If you tilt it to the right with the side buttons now at the bottom, it will stay in portrait mode.

Edit: ignore I was reading too fast and thought you were locked in portrait mode not landscape, sorry

Whats weird is the firefox mobile determines the orientation directly from accelerometer instead of just getting the orientation from the Meego API, so I don't think most of the tweaks that can lock orientation can have any effect on firefox.