Are you saying the ad opens when you start Firefox?

If it's like the fake FBI/Interpol browser lock page, This add-on can stop such pages; https://addons.mozilla.org/en-US/firefox/addon/disallow-script-button/ disallow Script Button

The Disallow Script button looks like a letter "M" and the title is the Minus Script, drag and drop the button on a toolbar. If the button is not displayed then nothing operates, except rules for plugins.

Another way out of the trap is to (force it if needed) Close Firefox. Wait about 20 seconds for Firefox to finish closing. Right-click on the Firefox icon and select Open New Window. +++++++++++++++++++++++++++++++++ Sometimes this works. If it opens, close it by Menu > File > Exit. This will flush out that web page from the Session Store.

Go to the Mozilla Add-ons Web Page {web Link} (There’s a lot of good stuff here) and search for a good ad / pop-up blocker.

Hello, thanks for your reply, I tried the link you post it and couldn't see the letter M as mentioned after restart my Firefox ... thanks again by the way the add knows who is my ISP since mention that when it open a second TAB beside my original TAB.my be one I take a screen shot of it .

Open the Add-ons Manager and make sure it is enabled. If you still do not see its icon,

Type about:customizing<enter> in the address bar. In the new window, look for the icon. When you find it, hold down the left button on it, and move it to where you want it to be. You can move any of the icons you want from here, but; some icons are locked in place.

There is also a Restore Defaults button at the bottom middle.

Note: about:customizing is no longer present in Firefox 47+ releases.

• Press Alt or F10 to bring up the toolbar.
  Then View > Toolbars > Customize.
• Right-click on a blank spot in the toolbar and
  select Customize.