I had to merge two different profiles in Thunderbird. I had the Portable Thunderbird on my thumb drive and the regular verison on my desktop. I had been using the portable version and was trying to merge my desktop profile with the portable. If you need to do something similar, I highly recommend against it. Apparently, the portable version does not allow it.
I tried and tried and it wouldn’t do it. I think I read the instruction on doing it many times over and over. The second I did it using the desktop version, it worked as expected.
Looking back, I am glad things worked out this way. It was a pain using the portable version. If I need to check email away from home, my ISP has made available the option to view my email on the web.