If you run a recent version of Firefox Dev or Nightly, and try to install an unsigned add-on, you may receive the error message that the browser prevented the operation from completing (... has prevented this site from installing an unverified add-on).
Also, if you have unsigned extensions installed in Firefox Nightly, you may have noticed that the most recent update disabled them.
Mozilla announced back in February that it would introduce a signing system for add-ons in the near future.
The system would prevent the installation of unsigned add-ons in Firefox Stable and Beta versions to better protect users from malicious and unwanted extension installations.
The decision was made to add override switches to Dev and Nightly versions of Firefox only so that unsigned extensions could still be installed in those versions of Firefox.
This is especially important for add-on developers but also for users who require add-ons that are not signed by Mozilla.
Mozilla's current plan is to introduce warnings with Firefox 40, enforce signed add-ons in Firefox 41 with a disable option, and remove that preference again in Firefox 42. Starting with Firefox 42, Stable and Beta users won't be able to override this anymore which means that unsigned extensions cannot be installed in those browsers anymore.
Nightly users of Firefox may have noticed that the browser blocks the installation of unsigned extensions as well currently.
Mozilla did not make it clear previously that it would enforce add-on signing for Dev and Nightly versions of Firefox as well but this appears to be the case as the installation of unsigned add-ons is currently blocked in Firefox Nightly (currently at version 42).
If you try to install the popular HTTPS Everywhere extension currently from the EFF website, you will receive the error message that Nightly prevented the installation of the unverified add-on.
It is unclear why Mozilla is using the term unverified in the error message and not unsigned, as signed is used on the Firefox add-on repository to mark compatible add-ons.
To fix the issue, do the following (please note once again that this will work in all versions of Firefox until Firefox 42 is released. When that release hits, only Dev and Nightly versions support the switch).
You may install unsigned extensions again in Firefox once you have set the preference to false, a restart of the browser is not necessary.
You will still get a warning message when you try to install unsigned add-ons, but the install button is there again so that it can be installed after all.
The warning reads:
Caution: This site would like to install an unverified add-on in Nightly. Proceed at your own risk.
It may come as a surprise to Firefox Nightly users that Mozilla set the default preference of the browser in regards to requiring signed add-ons to true.
While it is easy enough to modify so that this is not the case anymore, it will likely irritate quite a few users and especially those who run unsigned add-ons in Firefox.
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