The setting for new tabs is hidden. Here's how to change it:

(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 snapdo, 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?

If this change arrived unexpectedly, it could have been caused by downloading some free software that bundled in unwanted extras. I suggest visiting the Windows Control Panel, Uninstall a Program, and clicking the "Installed on" column heading to bring the most recent additions to the top. This may help uncover what rode in with what.

See also:

• http://malwarefixes.com/remove-snap-do-from-internet-browser/