You don’t need a plugin to do this any more, there are just a few simple steps to follow…

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste newtab and pause while the list is filtered

(3) Double-click the browser.newtab.url preference and enter the FULL url to your desired page:

Done.

Resource: https://support.mozilla.org/en-US/questions/930428