Jhieminga I was disapointed by the way the basic Firefox works : no provision to open a bookmark by default in a new tab 'à la Opera'.
The extension you mentioned (pedantic : it's called an extension, not a plugin) still doesn't help.
A bit of searching in the "hints and tips" yields this one :
Tab Mix which apparently does the job nicely
This extension does not change the "Tools/Options" menu, options for the extension have to be accessed through "Tools/Extensions" then Options in the dialog box.