In the resulting window ( Figure A), look for the line Profile Directory. To do this, open Firefox and type about:support in the address bar. The first thing we must do is locate our Firefox profile directory. How to locate your Firefox profile directory The only thing you'll need is a running instance of Firefox.
I'll be demonstrating on Linux, but this can be done on macOS and Windows as well (you just have to adjust the directories and tools used). I'm going to show you how to do just that. It's not 100% foolproof, but it has helped me in the past. One way to avoid such frustration is to regularly back up a specific directory which houses the session state. If I don't close that secondary window first when I reopen Firefox the next day, it will remember whatever state it was in when it was last closed. Let's say, for example, I forget that I had a second Firefox window open with a single tab. You see, I have Firefox set to Restore previous session (in Settings), so it always picks up where I left off.