As per the screenshot in Comment 4 from https://bugzilla.mozilla.org/show_bug.cgi?id=1595865 I have installed the HTTPS Everywhere extension on the latest Nightly and checked the existance of the
firstname.lastname@example.org in about:config, however, it is not listed there.
I have then proceeded to install several other extensions which might take control of preferences, such as New Tab Override and Ghostery, and could not find any specific evidence that control over some prefs is taken over by the extensions i.e. the ID of the extension is mentioned in the pref name as in the above mentioned screenshot.
The only prefs that have the IDs of the extensions in their names are from this specific category:
extensions.webextensions.ExtensionStorageIDB.migrated. . However this is the case for the latest Release version as well and not specific to FX73 where the fix should be present.
Is there a specific way to test this fix? And if so could you please provide some STR and possibly an extension to test this with? Thanks!