Features:

adds a context menu entry to
- unload the current loaded tab
- unload all but the current tab
avoids loading tabs when the active tab is closed (see the installation notice for details)
adds shortcuts to jump to the next/previous loaded tab
- the shortcut can be changed (in Firefox 60+)
compatible with
- Tree Style Tab: adds context menus and styles to the sidebar (see options)
- Container Tabs: unloaded tabs stay in the correct container
- Hidden Tabs (e.g. Tab Groups): works only on currently visible tabs
makes clear which tabs are loaded by graying out unloaded tabs
- this currently requires manual setup, see the installation notice
can be removed without leaving anything behind or closing unloaded tabs --> just try it

BUGS: Firefox' currently suffers from a number of bugs related to unloading tabs. Most notably, tabs may loose their icons or display as loading. Fixing that is Mozillas job. If you encounter any other problems, please report them as an issue or comment on an existing issue matching your problem. Please don't complain in a rating on the Add-ons download page. I can't respond to those. Permissions used:

Display notifications: Only to report errors
Access browser tabs: This should be pretty obvious