Left-clicking tabs sometimes opens that tab in a new window (without dragging)

I'm currently using the latest Firefox Nightly on Windows 10 LTSC 2019.

No special configuration that I can think of that would cause this, but when I used Tree Style Tabs, I also experienced this issue.

Tab Center Redux does not suffer from this issue. I assume this has something to do with the changed dragging code (you can drag tabs outside the window to open them in a new window in Reborn).

It happens pretty often and I can reproduce it with my config (chrome folder/user.js on a new profile; which can be provided), usually happening after clicking a new tab in the sidebar that was recently created by opening a link in a new tab with the context menu.