Race condition? Sent tabs loaded before STG loads are lost

[madb1lly]

Describe the bug
When I load Firefox, sometimes tabs arrive from other devices (via the Firefox Send Tabs function) and are added to the browser window before the rest of the tabs from the previous session are loaded with STG. When STG loads, the previous session is restored and the newly arrived tabs disappear, lost forever (at least, I can't find any way to get them back, e.g. reopen previous tab doesn't work).

Note that this I cannot reliably reproduce this error, but it has happened more than once.

To Reproduce
Steps to reproduce the behavior:

1. Have Windows computer with Firefox installed, STG installed and restore previous session enabled
2. On another device, e.g. Firefox for Android, send a tab to the Firefox instance running on the Windows computer
3. Turn on the computer, let it boot up and log into Windows
4. Open Firefox

Expected behavior
The previous session should restore and the tabs sent from the other device appear alongside the previous tabs

Desktop (please complete the following information):

• OS and version: [e.g. Windows] Windows 10 latest release I believe (I'm not writing this ticket from that computer)
• Firefox version: [e.g. 63] Latest stable
• Simple Tab Groups version: [e.g. 3.0.8] Latest

Additional context
I have a lot of tabs and several addons.