Support firefox's preferences

[directionless]

Firefox stores its preferences in a prefs.js file. Despite the name, this is not javascript. It would be nice to be able to parse these.

Some info:

• https://blog.mozilla.org/nnethercote/2018/03/09/a-new-preferences-parser-for-firefox/
• https://searchfox.org/mozilla-central/source/modules/libpref/parser/src/lib.rs
• https://firefox-source-docs.mozilla.org/modules/libpref/index.html
• https://github.com/hansmi/go-mozpref