There's actually a separate, hidden setting for new tabs: they don't automatically follow your home page setting.

(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 search box above the list, type or paste newtab and pause while the list is filtered

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

• Page thumbnails (default) => about:newtab
• Blank tab => about:blank
• Built-in Firefox home page => about:home
• Any other page => full URL to the page

Press Ctrl+t to open a new tab and verify that it worked. Fixed?

Two gotchas:

• If Firefox ignores the setting, one of your extensions may be overriding it. You can review, disable, and/or remove extensions on the add-ons page:
  
  Tools menu > Add-ons > in the left column click Extensions

• If the change works but at the next startup is back to Google, you might have a user.js file in your personal Firefox settings folder (your Firefox profile folder). This article describes how to track down and remove the file: How to fix preferences that won't save.

Any luck?

Please update to the current Firefox 25 release.

• Help > About Firefox

• https://support.mozilla.org/kb/update-firefox-latest-version.
• https://support.mozilla.org/kb/Updating+Firefox

The Firefox 19 version that you currently run is no longer supported with security updates.

• It is important to update Firefox and add-ons to the latest version to get all security fixes.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes