Contact the Addon creator for support using the Firefox addon page if they are listed there and if not then you need to search for them online.

''<p>info12810 [[#question-1502246|said]]</p>'' <blockquote>hi, for some reson my serialnumber for video downloadhelper does not work any longer? I am not sure you are the one to contact but at least it's a start, you may know who to mail in that case? </blockquote> On https://addons.mozilla.org/firefox/addon/video-downloadhelper/ there is a Support site link which leads to https://github.com/aclap-dev/video-downloadhelper/discussions and on top there is a "How to ask for help about licenses and purchase" link which is https://github.com/aclap-dev/video-downloadhelper/discussions/2 which then has a link https://github.com/aclap-dev/video-downloadhelper/wiki/I-need-help#license-problem