The browser's UI is closed source like much of Nokia's stuff on the N900, so you wouldn't be able to modify it (even though it'd just consist of removing a call to gtk_window_fullscreen).

It is possible to change the fullscreen state without the browser realising it, thus keeping it in portrait mode with the specialised toolbar using wmctrt; but it's annoyng to obtain the XID each time. You could automate this by getting the root window and setting a filter function (GDK offers such functionality) to monitor the _MB_CURRENT_APP_WINDOW property so that you could act upon that.
A rather convoluted workaround for something that Nokia should just open source the program for, however (see how removing one call would be easier?).

Just use Opera, which has a proper portrait implementation.

Another tip: when you want to close a browser window in portrait mode, press the rightmost button on the bottom of the screen (bookmarks), then the leftmost button on the screen top (dashboard), then when in the dashboard close the bookmarks window with its X handle and go back to your original window.
It's shorter to do than to tell

I highly recommend this method.

The reason I prefer the method I reported to pushing the camera button is that, when in portrait mode and kept by my right hand only, I feel easier to push buttons on the screen with my thumb than search the camera button, which for my own hand carries a serious risk of letting the thing drop on the floor