Andy McKay, Engineering Manager at Mozilla, announcedyesterday on the official add-ons blog that Mozilla would implement temporary add-on loading in its Firefox web browser.
Now why would the organization do such a thing? One major change to the add-on system of Firefox that hit Firefox 43 was the add-on signing enforcement.
All versions of Firefox ship with a switch that allows users to disable the enforcement so that they can load and run unsigned extensions in the browser. This will change in the future (likely Firefox 45) for Firefox Stable and Beta versions as Mozilla plans to remove that option from those browsers.

The change impacts not only users of the browser but also add-on developers. Considering that testing is a large part of the add-on creation process, not being able to test unsigned add-ons in Firefox Stable or Beta would seriously hinder and delay the development process.
Firefox Load Temporary add-ons

Temporary add-on loading is Mozilla's solution to the problem. Starting with Firefox 45, Firefox will support an option to load an add-on temporarily in the browser.
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The feature is aimed at developers who need to test their add-ons on Firefox Stable and Beta versions, but can be used by anyone using Firefox.

To load temporary add-ons in Firefox, do the following:

  1. Load about:debugging#addons in the Firefox address bar.
  2. Click on the "Load Temporary Add-on" button on the page that opens.
  3. Select a Firefox .xpi file from the local system that you want to load and click open afterwards. You can load a file from the add-on directory instead to load the extension.

The selected add-on is added to Firefox automatically without installation prompt. It is listed under Extensions on the page, and will remain enabled for the length of the browsing session. Once you restart the browser, it is removed automatically again and won't be available anymore until you load it again in the browser.

The new temporary add-on feature bypasses the add-on signing restrictions that are in place in Firefox allowing you to load unsigned extensions in the browser.

Since add-ons are loaded only for the session, it is not really of use to Firefox users who want to keep on using unsigned extensions without switching to Firefox Developer Edition or Firefox Nightly.

One big issue of the approach that Mozilla has not mentioned is that the method won't work for add-ons that require a restart of the browser.

The solution is not ideal, and if Mozilla would look over at how Google handles these things, it should probably consider bringing Google's Developer Mode feature to Firefox instead. This would be far more developer and user-friendly than what is created right now.