Auto Unload is disabled by default, you can enable it in extension options page.
Reasons for permissions required by extension:
Access your data for all websites: This permission is required to change favicons of tabs before unloading them.
Read and modify bookmarks: In Firefox 63+ you can right click bookmarks and open bookmarks in a unloaded tab. To do this extension require permission to read bookmarks.
Access browser tabs: Extension need to know URLs of each tab so it won't unload tab with site that is in whitelist.
