Extension pops up the onboarding window on every start of Firefox

[jakobbuis]

Steps to reproduce

1. Install extension
2. Close Firefox
3. Reopen Firefox
4. Close Firefox
5. Reopen Firefox

Expected: no window to configure the second time
Happened: windows pops up every time

This mainly affects my second machine: I have a work machine on which the extension is configured, and a second machine that is not used with the Remarkable and I'm not configuring it with the connector. But Firefox extension sync installs the extension nonetheless. It should only do that the very first time around after installation (if ever).