Priority 1 (highest)

Priority 2
Use ffmpeg to convert unsuitable video files (how do we check this?).
Add support for videos in preview mode.
Support more than 99 titles somehow (grouping them into titlesets won't help)
Track down and fix/suppress the NS_BINDING_ABORTED (0x804b002) error that occasionally appears in preview mode.

Priority 3 (lowest)
Provide a means to specify menu & audio language.
For any page with no links, add a "back" link (this raises the issue of localisation, of course).
If a page is too large, split it and add "next" and "previous" links (seems very difficult).
Set Mozilla's idea of the pixel aspect ratio to match video pixels. (For extra credit, scale preview mode to correct for the differing aspect ratio.) This seems to be impossible.
Avoid running Xvfb if we're only going to display an error message.
Allow explicit ordering of titles.