Read https://blog.mozilla.org/addons/2015/02 ... xperience/

Mozilla wrote:In the case of developers who want their extensions to be side loaded (installed via an application installer rather than using the usual Web install method) the review bar will be higher, equal to fully reviewed add-ons on AMO (with the exception of AMO content restrictions). This is a convenient installation avenue for software that comes bundled with an extension, for example an antivirus application that includes a Firefox extension that interacts with it.
(...)
The plan is to have this system working, at least in the transition stage, in Q2 this year. So, we will probably introduce extension signing warnings on Firefox 39.

I don't know how exactly the signing will work but it may be impossible to manually edit install.rdf file in the signed extension.

Current state of Mozilla Extension Signing is described here: https://wiki.mozilla.org/Addons/Extension_Signing

I'm afraid the tests with signing have already started, see here:
viewtopic.php?f=1&t=17249
viewtopic.php?f=1&t=16856&p=40803#p40803

Thank you, we're working on this.

Dear All,

We're glad to introduce the new FDM5 version that is fully compatible with new Firefox 41+ and, therefore, comes with the signed extension for the browser. You can read more and download the client in our blog.

As there were many problems reported with Mozilla Extension Signing, Mozilla developers decided to precise timeline for extension signing, see link in the first message:
* Firefox 40-42: Firefox warns about signatures but doesn't enforce them.
* Firefox 43: Firefox will have a preference that allows signature enforcement to be disabled (xpinstall.signatures.required in about:config).
* Firefox 44: Release and Beta versions of Firefox will not allow unsigned extensions to be installed, with no override. Signature checking is forced even in developers builds and it possibly should be disabled manually after each Firefox update.

See also older messages for more details about problems with digital signing:
* Checking signature for extensions and downloads
* Option xpinstall.signatures.required in Firefox
* Manual editing may block extensions