Open Multiple Bookmarks at Once in Firefox:

If you would like to open a number of bookmarks at once, without setting them as multiple home pages, try this: Take the group of bookmarks you want to open at once, and store them in one folder on the Bookmarks toolbar. Right-click the folder and select “Open In Tabs.”

Conversely, to save all of your open tabs at once, just click on the BOOKMARK menu and select BOOKMARK THIS PAGE. Though the window that pops up will prompt you for a name for the single bookmark, just pretend it’s asking you for a folder name. Type that in, select the Bookmarks folder for a location, and make sure you click “Bookmark all tabs in a folder.” A new folder will appear in your Bookmarks menu containing all the tabs currently open in your browser.