My goal was for every book that was canon set in the same universe . <| sep |> This link belongs here Differences between WebExtensions and X U L U S A d d o n s W e b E x t e n s i o n s C o m p a r i s o n w i thXULXPCOMextensionsQuote of summary At a very basic level , XULXPCOM extensions are similar to WebExtensions . They both include manifest files defining metadata for the addon and some aspects of its behavior . JavaScript code that gets access to a set of privileged JavaScript APIs and that stays loaded for as long as the addon itself is enabled . the ability to add specific UI elements , such as buttons , to the browser . Beyond that , though , the systems are very different . In particular Compared with XULXPCOM extensions , WebExtensions provide much more limited options for the addon ' s UI , and a much more limited set of privileged JavaScript APIs . WebExtensions can only access web content by injecting separate scripts into web pages and communicating with them using a messaging API note , though , that this is also true of XULXPCOM extensions that expect to work with multiprocess Firefox . This final point the need for a web page to inject the scripts into is of particular concern to me it seems to imply that download managers such as DownThemAll will simply be impossible to implement on the new system . It ' s encouraging that they note DownThemAll as an interesting addon , but it ' s currently rather unclear how its basic MO can be reconciled with the above .