True, but that's just trying to make it easier for the developer, not the user.
Not just that, but like you said, you have to *learn* the action. Once you've learned it, the icons are only reminders for something you already know what to do.
The most important use here is teaching new users new interactions the first time using it, and these icons aren't doing the job.
Tap on the top-most bar (with the time/network/etc. on it). Works system-wide.
Funny enough, I JUST figured this out today and it has a lot to do with ergonomics. It's because when I am scrolling up, my swipes are not as straight as when they are going down.
Simultaneously, Fennec does NOT take into consideration that most people can't pefectly drag straight up and down -- you can see this when you try to flick-scroll when zoomed in: the page will slightly shift left or right at the same time (being right-handed, it's to the right for me). It's impossible to drag a page down without shifting it slightly.
BTW, the original Mac OS HIG has a lot to stay about how menu panels are triggered and kept open on a curve because users don't ever move on perfect vert/horiz paths. If you've ever had a hard time moving from menu panel A to menu sub-panel B, it's because someone was too strict about that movement and didn't allow room for error.
There's definitely a lesson for Fennec to learn there if they want to make the flick-scrolling feel natural since they've added these left and right panels to dragging a page around.