User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0

Steps to reproduce:

Running Mozilla Firefox Nightly 78.0a1 20200519160549 20200519160549 on 64-bit Fedora 32, I went to check for updates as I do every day.

Actual results:

got
"Update failed. Download the latest version"

Expected results:

Successfully updates.

@FirefoxNightly Twitter account says
"PSA: our last desktop nightly had a bug which prevented websites from loading if you had extensions installed (https://bugzilla.mozilla.org/1639429). The faulty patch was backed out and new nightlies built, you can update now."

but this is very confusing.

1. bug 1639429 is nothing to do with updates besides a comment "Nightly updates are frozen to the nightly prior to this regression (20200519094847)."
2. I still can't update.

I don't know if you'll ever be able to do a regular update from my version. The workaround is, as Nightly says, "Download a fresh copy of Nightly."

I filed this bug because this issue is un-find-able unless you're on Twitter and the explanations are unclear.

Bugbug thinks this bug should belong to this component, but please revert this change in case of error.

Bug 1639979 is perhaps more informative, you may be able to work around it by going into about:config and setting security.cancel_non_local_systemprincipal to false. It isn't clear that addons are really related.