Thank you for your review! I have just released a new version (2.10.0) that adds support for "chrome://" URLs for the quick bookmarking icon.
Additionally, the URLs starting with "file:", "about:" and "moz-extension:" are also supported on top of the classic "http:" and "https:".
Please note the update may take a bit of time to be available to everyone.
If you want to report another issue or request a new feature, please feel welcome to do so on GitHub at https://github.com/teddy-gustiaux/default-bookmark-folder/issues.
The Mozilla Add-Ons website only allows me to respond once to a review, so it is not the ideal place for a conversation. :)

Thank you for your review! :) The signature issue is an internal Firefox issue that is affecting all extensions. Mozilla is actively working into fixing the problem. You can follow their progress on the dedicated forum thread (https://discourse.mozilla.org/t/certificate-issue-causing-add-ons-to-be-disabled-or-fail-to-install/39047) and on the bug report (https://bugzilla.mozilla.org/show_bug.cgi?id=1548973).
EDIT: The problem has been fixed with the release of Firefox 66.0.5. Please see the dedicated blog post (https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/) for more details or if you are using an older version (52 to 56) of Firefox.