I have an add on called LeechBlock that blocks websites. https://addons.mozilla.org/en-US/firefox/addon/leechblock/

One possibility is that you have an add-on which is causing this problem. You could review your add-ons and disable everthing non-essential, then restart Firefox and see whether the problem stops.

orange Firefox button or classic Tools menu > Add-ons

Review both the Plugins and Extensions categories. If anything appears to be suspicious, you could go ahead and remove it rather than disabling it.

Does that make any difference?

In case the cause is external to Firefox, you may want to run some supplemental security scans. These two tools are highly regarded:

Malwarebytes Anti-malware : http://www.malwarebytes.org/products/malwarebytes_free

SUPERAntiSpyware : http://www.superantispyware.com/

Edit: If this is something implemented by your service provider, then the above would not be relevant.

Some people use the Windows hosts file to prevent loading of annoying sites. The general idea is that you enter the host name you want to block, say, doubleclick.net, with an address that points back to your own system. Then when Firefox is directed to request something from that host, nothing loads.

For more information on the Windows hosts file, see: http://en.wikipedia.org/wiki/Hosts_%28file%29

Thanks Jscher, but can you tell me what I should do exactly? I found the hosts file opening in WordPad program based on what I conceived from Wikipedia but don't know what should be written there. I guess the problem is not of my desktop cause it appears to other systems using the same router. yours

Here's an analogy: let's say the site you want to block is annoyingsite.com. In the hosts file, you would enter

127.0.0.1 annoyingsite.com

to redirect all requests for the annoying site to your own computer, which prevents them from being retrieved from the real server.

Thanks mormi, I installed the Add-on, and it works ,but the problem is that the annoying tabs are still opening repeatedly in a new page I introduced to the add-on--yahoo.com. It's because among the options there is not one which helps me prevent any new extra tab from opening

So it's different sites that open tabs and new ones every time? Yeah my option is just like jscher's - just blocks specific ones. I could have sworn in an earlier version of Firefox there was a banner that told you that a page wanted to open a pop-up or another tab and asked if you wanted to allow it. They shouldn't have gotten rid of that, it seems.