This mainly concerns the landing page, but also download pages.

Critical info

1. Explain what exactly DevEdition is. It's similar to Beta, except that installing Beta is potentially dangerous as it uses the same path as GA Firefox, thus overriding it with a new profile and making it hard to find the user's original profile. DevEdition gets around this with a clean experience which is perfect for experimenting.
2. Include up-to-date info on what features are DevEdition-only. For example, we are letting WebSocket and Multiline Editor sit for at least one release in DevEdition and not Beta.
3. Consider what is important to bring over from the docs. Different default preferences, themes, etc.

For the first milestone of this project, I'd like to replace this section with a better description:

@violasong it would be great if we could also add the accessibility panel to the page. it's missing from the tool overview at the bottom.