Could you share what the command was?
Erm.. it was a move command.mv -v /path/to/old/ffprofile.name /home/username/.mozilla/firefox/ffprofile.name
In profiles.ini, change the profile name to the one you just moved. The next time you start firefox you'll have your settings, bookmarks and everything else back to how you left it when you last worked with this profile.
Well, I must have had some messed up settings due to one or more of the many extensions I have installed in Firefox on Windows. I did all this and it simply would not work in Linux for some reason.
Nonetheless, I thank you for sharing. (I was hoping there was a command I did not know about that might fix this next time I need to copy one of my profiles over.) But as I am now using Waterfox on Windows, I suspect it may not be possible to copy or merge profiles with Firefox on Linux. Oh well. It is still worth the trouble.