Review some extensions.* preferences

There are a few extension preferences that confuse me:

extensions.databaseSchema
extensions.enabledScopes (cited in the design document)
extensions.pendingOperations: Firefox uses for its own purposes and it changes it whenever it needs
extensions.ui.lastCategory: it's used for navigation purposes, or to open the last visited category in about:addons.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information