I think the best solution to provide the ability you're looking for is a Firefox add-on called NewTabURL:
NewTabURL (https://addons.mozilla.org/en-US/firefox/addon/2221/)
- Allows you to choose the default URL to open automatically in a new tab; a page of
your choosing, your Firefox homepage(s), or a blank page.
The two options Idigas mentioned behave a bit differently and may not have what you are looking for. New Tab Homepage loads your homepages(s) automatically in the new tab you open (which may not be the URL you want in the new tab). New Tab King opens up a menu of frequently used URL's, recently closed tabs, suggested sites. It has a lot of other doodads and configuration options such as sidebars, popup menus, etc. but I don't think you can just pick one URL for it to automatically load in a new tab.
Hope this helps,
CFW