There's nothing built into Firefox to do that from a left-click, but middle-click or { Ctrl + LeftClick } will open a bookmark in a new Tab.

Or you can install an extension which adds that feature to Firefox, both Tab Mix Plus or Tab Utilities extensions have that feature.

• Open Bookmarks in New Tab: https://addons.mozilla.org/firefox/addon/open-bookmarks-in-new-tab/
• Tab Utilities Lite: https://addons.mozilla.org/firefox/addon/tab-utilities-lite/

Keyboard shortcut is built into Firefox there is no need for an extension, but you will have to make a modification perhaps to a configuration variable.

set the following configuration variable

 browser.tabs.loadBookmarksInBackground

to the same value as you have for

  browser.tabs.loadInBackground

Opening links in the background with Ctrl+Shift+click or foreground with Ctrl+click or the other way around is controlled by Tools > Options Tabs > when I open a link in a new tab switch to it immediately.

More information on configuration variables available in about:config entries and for users not familiar with the process there is about:config (How to change).