Please be informed the tutorial below DOESN'T WORK anymore.
With Firefox 41 (and higher), you cannot customize the page shown in a new tab by changing the preference browser.newtab.url in about.config.
But don't worry, it can be fixed by using this Firefox Add-on called New Tab Homepage.
1. Go to New Tab Homepage.
2. Click on the green color "Add to Firefox" button.
3. It will start downloading. Then click on the "Install" button.
4. That's all.
Now when you click on the new tab, it should open with Google homepage.
If your Firefox Home Page setting is set to Google, then when you click on the new tab, it will show you another same kind of Google homepage with its Google logo and the search box.
But if you have reset it (probably because it was attacked by malicious browser hijackers like My Start Search or Omiga Plus), you discovered that when you clicked on the new tab, it does not open the usual Google homepage as before anymore.
Don't fret. Let me me help you to fix it.
How to set your tabs to open with your home page
1. In the address bar at the top of the page, type about:config
2. Press "Enter" key.
3. A page will pop up with a warning sign:
"This might void your warranty!"
4. Just click on the "I'll be careful, I promise!" button
5. A page will appear with a list of application settings.
6. Now type browser.newtab.url into the search box above.
7. Immediately it will filter out the rest and show you only browser.newtab.url together with its status, type and value.
8. Put your cursor over the line and double click.
9. A box will pop up.
10. Delete away the text browser.newtab.url in the box.
11. Replace it with https://www.google.com
12. Then click "OK".
13. That's all. Now when you click on the Firefox new tab, it will open to your home page as you preferred.
Note: Now you will notice the URL is not just https://www.google.com.my, (for those in Malaysia) but together with some other strange-looking code added behind the URL.
It looks like this: https://www.google.com/?gws_rd=ssl
* Here is a tutorial on how to remove the additional code from the Google URL.