Okay, I uninstalled and installed the latest version using SDCentral (v1.11.518u Plus), and that seems to have fixed it. Yay!
Corky_0> First of all, I created the fly-out menu by right-clicking the dock itself and navigating to Add -> New Fly-out Menu. Then I defined the contents by right-clicking the menu icon and choosing Dock Entry Properties (which opened the usual window from within which I added all my first and second level entries). It took a little while to get everything in there. ; )
As for Firefox instances and physical memory - I'm on a 1gb RAM system, and I'm running a little app called FreeRAM which shows the available amount of RAM in a little tray icon. I check it often to see it's not too low (which would be an indicator of some of my running applications malfunctioning and leaking memory), so at the time of the screenshot I'd wager I had more than 400mb RAM available.
I don't have a similar easy way to keep track of video memory, though. I'm using an ATi Radeon 9800Pro card (128mb), so while I don't think the video memory would fill up very easily, nothing's saying it wasn't full.
Well, since it seems to be working in this latest beta, I'm pretty content. =P