As mentioned here: https://blog.mozilla.org/addons/2017/09/28/webextensions-in-firefox-57/.

This is a big deal for a blocker such as uBO.

One immediate use I want to evaluate is to be able to inject a site's scriptlets -- if any -- before any other inline script tags on the page. If uBO injects its scriptlets before all existing inline script tags in a document, this guarantees that the scriptlets win over these inline script tags.

For instance, if Chromium had this API, there would be no need for a separate extension to foil Instart Logic and other such nuisances.