Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
By default when you click the Open All in Tabs option in a bookmarks folder or a live bookmark, the bookmarks are opened in different tabs replacing the current tab set you have at that time.
According to what I’ve read, this was done for consistency as back, forward, search, link clicking and any other action that loads a new web page overwrites the current content by default, and getting it in a new tab requires a link modifier like Ctrl + clicking the back button, a bookmark or a link, or Alt + Enter-ing in the location or search bar.
While this is true, I don’t find it intuitive. To change this behavior and make Firefox append (add) the bookmarks to the current.