Thank you for your review! :)

Thank you for your feedback! You should have been redirected to the add-on settings page automatically when you installed the add-on.
It did not seem to be the case, however you can access the add-on settings by going to the Firefox add-ons manager (accessible via the menu "Tools" then "Add-ons" or by using Ctrl+Shift+A).
Then simply click on the "Options" button next to Default Bookmark Folder to get to the settings page.
Once there, you can set up the add-on to fit your needs!

If you have any other questions or problems, please feel free to create a new issue on GitHub (https://github.com/teddy-gustiaux/default-bookmark-folder/issues).
If you do not have an account, you can create one for free.
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 feedback! :) Sorry to hear about the icon issue you are experiencing.
If you can create a new issue on GitHub (https://github.com/teddy-gustiaux/default-bookmark-folder/issues) and provide me with some information (listed when you create an issue), I will be happy to look into this!

Thank you for your review and your feedback! I have updated the description of the add-on to include these features. :)

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.