There are a number of ways you can open a bookmark in a new tab from bookmarks menu: right click and select open in a new tab (this could be difficult for laptop users as they have to use touchpad), Ctrl+Clicking will do, middle clicking mouse button also opens the bookmark in a new tab and there is an extension Tab Mix Plus serves the purpose.

Mozilla has reopened a bug closed six years ago when asked to offer a pref to open bookmarks in a new tab. Firefox team closed that citing extension available with the same name (AMO link) then, which has now 34k users. This and TabMixPlus may not be available as WebExtensions when Firefox 57 releases.

Firefox 57 gets a preference to open bookmarks in a new tab

Considering this, Mozilla has decided to offer a preference  ‘browser.tabs.loadBookmarksInTabs in about:config. This pref is not available by default, you need to create it and change its value to true, here is how you can do that.

1. Visit about:config and proceed by agreeing to the risk button

2.  Right click New >Boolean, enter the preference name as ‘browser.tabs.loadBookmarksInTabs’

3.  Change its value as ‘true’, done.

Now visit bookmarks menu and  try to open bookmark with a single click it will open in a new tab.