On the n810, the volume buttons just sent keystrokes and we use those to zoom in and out. On the n900, it seems that the volume button are handling the volume by default. Although, the Maemo Browser seems to trick the buttons to do zooming.
I think we should close this bug, based on the comment here: https://bugzilla.mozilla.org/show_bug.cgi?id=478038#c16
sorry for bug spam. Many of the bugs which are marked invalid, I see comments telling it occurred in one version or other. But later it was fixed due to 1) by backingout the patch which made regression or 2) by fixing some other bug. So if we can identify the bug/patch/reason then we should state that and mark those as FIXED. If not mark as WORKSFORME in that case.