It kind of works.
Youtube changed my home page to show only 3 videos per row, and this addon fixes it. But I have to manually click the addon and change the settings everytime I open youtube.

This actually worked great for a while to counteract the terrible UI design YouTube has been implementing lately. However as of May 1, 2025 this extension no longer works for me. Not sure if it's due to a recent Firefox update or a recent YouTube update, but I'm back to the stupid 2 or 3 thumbnail across situation.

Update in response to developer response: Tried changing number of columns, refresh browser, quit and re-open browser and did not work. Another potential source of this add-on no longer working could be a problem with other extensions...