How to view extension privileges in Firefox?

Read this document. Relevant excerpt (page 3):

The Firefox platform has no mechanisms to restrict the privileges of add-ons. The add-on code is fully trusted by Firefox. The installation of malicious add-ons can result in full system compromise. There is no security measure to restrict the intercommunication between add-ons. As a result an add-on can alter or modify another add-on in the background. There is no security policy or sandboxing ability of XPConnect and XPCOM components which is a serious flaw in the security model. Firefox does not have any type of restrictions on malformed Cross Origin Resource Sharing and socket creation. Some exploitable vulnerabilities are platform independent.

So there is no way to show per add-on permissions because there are no permissions to show. In the Firefox security model, each add-on is roughly equivalent to a piece of native code, which can, by definition, do whatever it wants. This is similar to the situation with signed Java applets.

As of Firefox Quantum (v. 57+), there is an add-on, Project Insight, to show the permissions of other add-ons (and itself).

Note that it only displays active add-ons, so at least temporarily, enable all.

Permission Inspector add-on displays the permissions granted, including those for disabled add-ons. It also displays all extensions having a particlular permission.