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 lose their icons or display as loading. Fixing that is Mozilla's 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

Development builds are automatically created on every commit with appveyor and released on GitHub.
These builds use a different id (-dev suffix), so they are installed as an additional extension and do not replace the release version. This means that:

• you probably want to disable the release version while the development version is active
• any options set are managed individually (so pre-release versions can't mess with your settings)
• they never update to release versions, but
  • they update themselves to the latest development version
  • every release version has a corresponding development version (the one with the same prefix and highest build number)