Its important to have the volume reset, so you don't get blasted with super over amplified volume. Since most websites don't normalize volume, it's a big risk to save this setting.
It also requires me to request more API access from Mozilla. As that functionality would require "local data access" for the extension to remember your settings. Requiring more permissions, generally makes users more anxious about the extension and adds further scrutiny from Mozilla admins.
So I hope you can understand, why I haven't include this functionality...
Please amend your review to a higher rating, create an issue on the github repository, and I will make a release for you in the next month with this functionality.