This is discussed in: https://github.com/M-Reimer/undoclosetab/issues/19
The bug is inside Firefox and as I haven't managed to reproduce this, so far, I wasn't able to find a possible workaround. So if you at some time find some way to reproduce the issue, please post in the above GitHub issue.
Actually developing for Firefox is often all about finding proper workarounds for all the issues Mozilla has in their APIs which are usually not fixed with priority... :(