Open the browser console by hitting ctrl-shift-j
Copy and paste the following code, hit enter. Until mozilla fixes the problem you will need to redo this once every 24 hours:
    // Re-enable *all* extensions
    async function set_addons_as_signed() {
        Components.utils.import("resource://gre/modules/addons/XPIDatabase.jsm");
        Components.utils.import("resource://gre/modules/AddonManager.jsm");
        let addons = await XPIDatabase.getAddonList(a => true);
        for (let addon of addons) {
            // The add-on might have vanished, we'll catch that on the next startup
            if (!addon._sourceBundle.exists())
                continue;
            if( addon.signedState != AddonManager.SIGNEDSTATE_UNKNOWN )
                continue;
            addon.signedState = AddonManager.SIGNEDSTATE_NOT_REQUIRED;
            AddonManagerPrivate.callAddonListeners("onPropertyChanged",
                                                    addon.wrapper,
                                                    ["signedState"]);
            await XPIDatabase.updateAddonDisabledState(addon);
        }
        XPIDatabase.saveChanges();
    }
    set_addons_as_signed();