I think I found the preference responsible for the bug: browser.uiCustomization.state.
I’ve encountered this issue on Windows and not Mac OS, but I can confirm that deleting this pref solves the issue for me.
This was the value:
user_pref("browser.uiCustomization.state", "{\"placements\":{\"widget-overflow-fixed-list\":[]},\"seen\":[\"developer-button\",\"profiler-button\"],\"dirtyAreaCache\":[],\"currentVersion\":16,\"newElementCount\":0}");
If it helps, I can also send you my profile folder backup if there is anything else in there that would indicate more.