When Mozilla becomes aware of add-ons, plugins, or other third-party software that seriously compromises Firefox security, stability, or performance and meets certain criteria, the software may be blocked from general use. For more information, please read this support article.
The internet holds many explanations of remote code execution, Wikipedia has a general article about it. Remote code execution is explicitly forbidden as per our policies.
Mozilla runs a Recommended Extensions program. Recommended extensions are curated extensions that meet the highest standards of security, functionality, and user experience. Firefox staff thoroughly evaluate each extension before it receives Recommended status. Video downloaders are part of the program, I am sure you will find a suitable replacement there.