I worked from 10 years with imacros for firefox 56 and working well, never show this problem, after firefox migrate to new engine Quantum all old macro recorder need to rewrited in new code and there are many incompatibility.
Here some old post with the bug “Error #101: Kantu is not connected to a browser tab” read the date, i tried all ui vision version and sometimes show this error.
[Apr 16]
[Nov 18]
[Jan 4]
[Apr 4]
And more and more
In my opinion after 2 years with this error i think never will be fixed.