Starting with version 40, Firefox gives a warning on unsigned (by Mozilla) extensions, while starting with version 41 these extensions won't be allowed to install. There is no option in about:config or similar to disable this behaviour. People not wanting to have this behaviour would need to switch to the Dev-edition (which brings a lot of bloat) or use an unbranded build. As such, the given setting will probably be a compile-time flag for which I'd like to request a USE-flag. Ideally, default state should be signing enabled with a one-time einfo telling people about it. Reproducible: Always
As a heads up, this will not be made mandatory per current plans until firefox-44. Additionally, Firefox ESR 45 will not enforce the mandatory extension signing. See https://wiki.mozilla.org/Add-ons/Extension_Signing for more details.
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