In a somewhat controversial decision, it was decided that if you open a group of tabs (middle-click on a bookmark folder) in Firefox, those new pages will be opened in the currently opened tabs, rather than being added. There is, however, a setting to make groups of tabs be opened in new tabs, rather than the existing ones.
Enter about:config in the address bar. In the resulting page, search for the preference browser.tabs.loadFolderAndReplace.
Double-click on it, which should change the value to false.