After further analysis, it's working great on my desktop with Firefox (except Firefox still hasn't updated it from the cached one in my bookmarks toolbar). It's not working on Chromium, though (Chromium is a lot like Google Chrome). It's only partially working on my Kindle Fire's Silk browser (only the very main page has the favicon for it).
I notice there are no html5 macros. So, I don't know how to follow GregChapman's post about it, which says to add a line of code to the macro called html5_impl.
Okay, I figured out how to fix it. It wasn't Nabble's fault! :) I just needed to make an ico file with multiple sizes packed into it. I was using an svg (which I guess works with Firefox's tabs, but not all other browsers).
I also changed the rel attribute in the link tag in my macro from "shortcut icon" to just "icon".
So, now it shows up on my Kindle Fire, and my Firefox bookmark's toolbar even updated it after a minute or so.
It works on Chromium, too, but the background isn't transparent there for some reason. It is transparent on Firefox, but it's not on my Kindle Fire. Close enough, for now, but hopefully I'll figure out how to fix that.
Edit: I tried adding -y 0.0 to the inkscape call, but it's still not transparent on Chromium and my Kindle Fire. Alas. I also added an additional size, which I saw someone using: 180x180.