I keep sites grouped in the toolbar in FF and occasionally make the mistake of trying to use ‘open all in tabs’. This closes / replaces what I have open. I’m not sure exactly, there’s a lot of discussion about what it does do and what it should do. There’s a ton of discussion in bug 175124 and bug 258224 about what it should do. If you always remember to middle click on the folder, that works well enough for me. To change the action of open all in tabs however, which you can’t middle click on, change browser.tabs.loadFolderAndReplace to false in about:config (there’s no option in preferences afaik).