http://kb.mozillazine.org/Browser_starts_with_multiple_tabs_or_windows_open

I read that, but I do want Firefox to start with my previous tabs open. It has been doing that for me for the last few years. That article seems to deal with tabs, not separate copies of Firefox, even if that is what it considers "windows."

This is not really the problem. What I want to get rid of is the second copy or window of Firefox that has only begun to appear the last couple of days. In other words, I get two copies of Firefox when I open it now, one with my previous tabs open as I want it, and the other with my home page and any other (different) tabs I may have had open in this copy. There are two different copies of Firefox open, each with their different sets of tabs now. I only want one

You can check for problems with the sessionstore.js and sessionstore.bak files in the Firefox profile folder that store session data.

Rename (or delete) the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox profile folder.

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Multiple_profile_files_created

Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost and you will have to recreate them (make a note or bookmark them if possible).

Tried removing read-only attributes to files as the articles above suggested. There were no extra numbered files as it said there might be, though. Did not seem to help until I closed the second copy with the option to close all tabs. That simple action closed just that one copy and left me with the one I wanted still open. I closed it and reopened it, and it still gave me only one copy, so I guess everything is fixed.

Thanks to those who tried to help!