Firefox Quantum. does not allow extensions to choose the location of their menu so that is not possible. Each extension is only given 1 menu space at the bottom so every menu has to come under that 1 space.
You are Marking down the add-on for something that is not possible to do.

Please don't mark the add-on down for a feature request or support queries.
Please post all support questions/requests to the support topic.

The support site is listed on add-on's page under "Support Site". It is also listed on add-on's Help Page.