Since v41.0, new tabs in Mozilla Firefox automatically open in the thumbnails page (about:newtab). This is due to the fact that Mozilla has blocked the browser.newtab.url property which previously allowed users to open new tabs with their preferred URLs (homepage, Google, Bing...). This browser property was constantly being targeted by malwares, redirecting users to webpages containing more malwares or scams.
In order to open new tabs with a specific URL in Firefox 41.0, you will need to use a browser extension. Here's a small list of our favorite ones: