Edit:

1 - Trying in safe mode does not help. 2 - Disabling anti-virus does not help. 3 - Trying on Firefox standard works!

That's strange. They've been changing signatures recently (for extensions uploaded prior to April 2019; that one is newer), but it should be backward compatible. Wait to the next Beta, we'll see.

Will the beta show in the About dialog? If not, is this the one you mean:

https://download.mozilla.org/?product=firefox-nightly-stub&os=win&lang=en-US

The nightly allows this extension to be loaded. Thank, your suggestion prompt me to look for it!

"IMDb Search" extension installs for me on Developer Edition 126.0b5 and also with extensions.langpacks.signatures.required set to false.

Does it still happen with default settings in a new browser profile? Alternatively you could refresh your existing profile. Make sure you stay signed out of your sync account when testing.

Yes, it does happen with a new profile.

"IMDb Search" extension installs for me on Developer Edition 126.0b5 and also with extensions.langpacks.signatures.required set to false. Does it still happen with default settings in a new browser profile?

Yes, it does happen with a new profile. My Developer Edition version is also 126.0b5. Really weird, a new profile is indeed a blank slate. Although... ding ding! I have installed TAB MIX Plus, which requires 2 files in the FF install directory:

config.js:

// skip 1st line lockPref('xpinstall.signatures.required', false); lockPref('extensions.install_origins.enabled', false); try { const cmanifest = Services.dirsvc.get('UChrm', Ci.nsIFile); cmanifest.append('utils'); cmanifest.append('chrome.manifest'); Components.manager.QueryInterface(Ci.nsIComponentRegistrar).autoRegister(cmanifest); const objRef = ChromeUtils.import('resource://gre/modules/addons/XPIDatabase.jsm'); objRef.XPIDatabase.SIGNED_TYPES.delete('extension'); ChromeUtils.import('chrome://userchromejs/content/BootstrapLoader.jsm'); } catch (ex) { /* empty */ } try { ChromeUtils.import('chrome://userchromejs/content/userChrome.jsm'); } catch (ex) { /* empty */ }

C:\Program Files\Firefox Developer Edition\defaults\pref\channel-prefs.js

/* This Source Code Form is subject to the terms of the Mozilla Public * License, v. 2.0. If a copy of the MPL was not distributed with this * file, You can obtain one at http://mozilla.org/MPL/2.0/. */ // // This pref is in its own file for complex reasons. See the comment in // browser/app/Makefile.in, bug 756325, and bug 1431342 for details. Do not add // other prefs to this file. pref("app.update.channel", "aurora");

C:\Program Files\Firefox Developer Edition\defaults\pref\config-prefs.js

pref('general.config.obscure_value', 0); pref('general.config.filename', 'config.js'); pref('general.config.sandbox_enabled', false);

Removing them allows to install IMDb, and re-installing them enables TAB MIX Plus again, without disabling IMDb.

Interesting.

The TMP team offered a solution here:

https://github.com/onemen/TabMixPlus/discussions/292#discussioncomment-9250738