Firefox 42 will not allow unsigned extensions

It's important to note that the Developers Editions (and the Nightlys) will have a setting for disabling the requirement.

The assumption being that developers need to test as they develop. And are a more informed user.

The other assumption being that there is a distinction between 'developers' and 'users'.

Did they say why beta wouldn't have this setting? If anything beta is closer to release and developer would target that. Developer edition is still nightly if I'm not correct?

I don't see anything about why the beta, but I did see the following in the FAQ. Maybe this will help:

"There will also be special unbranded versions of Release and Beta that will have this setting, so that add-on developers can work on their add-ons without having to sign every build."

The link also says that there will be builds of normal (release) and beta Firefox that do not have this limitation, for those that want them.

(In addition to people always being able to recompile the browser with whatever modifications they want, of course.)

I had to flip that setting this morning when dev edition updated and disabled the 1Password extension. It's "xpinstall.signatures.required", for reference.

They've said that for a while, but the last time I had the Dev edition installed I couldn't install the Dashlane [1] plugin.

1: https://www.dashlane.com/en/cs/3bce5a89

I think they want to encourage wider adoption of the Beta version, so they treat it similarly to the Release. The logic seems pretty questionable to me though. If you can install a Beta version of Firefox, you should be able to avoid consenting to allow malware to run on your computer (this change is primarily targeted at extensions installed by some user action -- like something silently side-loaded by an application installer).

Every user should be permitted to disable that requirement: it's his browser, not Mozilla's.

Generally, beta is supposed to be almost completely identical to the release version, to ensure that what gets shipped to release users is tested. This particular pref seems harmless, but you never know.

Developer edition is what used to be known as "Aurora", which is in between Beta and Nightly.

I was only commenting on the "trival build or not" part. You're right that there are plans to have official "unofficial" release and beta builds without signing requirements, but only for the en-US locale (yes, language packs exist, no, not every developer on Earth speaks English)