I've been testing my Firefox OS app for ownCloud in the FxOS simulator that comes with the Firefox App Manager. While the simulator replicates the majority of FxOS functionality fairly accurately, I did run into a small hiccup that frustrated me endlessly until I tracked down its source.

The FxOS simulator sometimes needs a little help to configure stand-in media directories in Linux. In order to simulate dedicated media storage on mobile devices, the FxOS simulator attempts to mount local media storage directories on the host computer (with "~/Pictures" standing in for the phone's photo storage, et cetera). However, certain Linux distributions may need to have additional packages installed before the Firefox OS simulator can locate those directories (including, as it turns out, Ubuntu 14.04 plus the KDE, which I would imagine is a fairly common setup, especially in the ownCloud community). I don't fully understand how this discrepancy developed, but installing xdg-user-dirs from the Ubuntu repository and then running xdg-user-dirs-update resolved the issue. You can read more about xdg-user-dirs on the FreeDesktop site. Much thanks to Matt for helping me figure out this mystery.