Try going to the top of your Firefox page under EDIT then scroll down to PREFERENCES> IN the GENERAL tab where it says "when Firefox starts use home page"....

just under that there are choices.  

Choose "restore to default" That should get rid of the problem. If not, Write back and we'll try again

This can be caused by an add-on, either one that you selected unaware of its side effects, or one that snuck onto your system as part of a bundle with something you actually wanted.

Try this:

Disable ALL nonessential or unrecognized extensions on the Add-ons page. Either:

• Ctrl+Shift+a
• orange Firefox button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, if in doubt, disable (or remove).

Usually a link will appear above at least one disabled (or removed) extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Hopefully that will help shut it down.

You also should:

(1) Visit the Windows Control Panel, Add and Remove Programs, click the Installed On column heading and cast a critical eye on everything recently installed for potential removal.

(2) Perform a few malware sweeps for items that aren't listed as Extensions or in Add/Remove Programs but may be undesirable. See: Troubleshoot Firefox issues caused by malware.

Or, if this is appearing when you open a new tab (click the "+" button or Ctrl+t) inside the tab, there's a hidden setting to fix that.

(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:
  
  orange Firefox button (or Tools menu) > Add-ons > in the left column click Extensions

• If the change works but at the next startup is back to the unwanted site, 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?

Its more than likely that you got malware. I would recommend scanning PC with Malwarebytes, Hitman pro and adwcleaner too.

I've been having the same problem for some time. Very annoying, showing with both FireFox and IE. Looks like it is caused by the Shockwave Flash plugin, which is trying to load a 1-pixel file "sc.gif" into a new window. I have absolutely no idea how can such a badly bugged plugin even make it to the market and WHY IS THIS ISSUE NOT ADDRESSED YET??? Any quick solutions? Thank you.

Hi sim_e_on, as a first step, could you try the post the original questioner marked as the solution? https://support.mozilla.org/en-US/questions/981953#answer-517513