Discards pinned tabs on browser startup, so tab sidebar extensions like Tree Style Tab and Sidebery show them in the unloaded (dimmed) state instead of appearing loaded.
Firefox restores pinned tabs as loaded on startup, even with browser.sessionstore.restore_pinned_tabs_on_demand set to true (Bugzilla 1466440, 1853047).
On startup, the extension discards every pinned tab except the active one. This restores the dimmed look in TST/Sidebery and frees their memory until visited.
