We've had several reports of pinned tabs failing to be restored by session restore in today's Firefox Nightly.
For me, this happened twice, with different sets of pinned tabs failing to restore:
(1) After restarting Firefox to install an update, gmail + gcal tabs for MoCo account (but personal gmail + gcal were fine)
(2) After restarting Firefox just for the heck of it (no update), gcal tab for MoCo account & my MoCo slack tab failed to restore.
The failed-to-restore tabs just have the gray "oscillating ball" loading animation, and they stay in that state forever.
The only workaround I've found so far is to close the tab (e.g. middle-click the tab title) and then do undo-close-tab with Ctrl+Shift+T.
(Note: I performed this workaround between instances (1) & (2) above -- so the tabs that failed to restore in (2) were "newly" failing to restore. It's not that they were in a still-broken-from-before state when I got to instance (2) of the issue.)