you should use this addon: https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/

hello, there are adware extensions in your profile - please try to reset firefox and see if this can address the issue...

Troubleshoot Firefox issues caused by malware

You have malware/adware. Reset Firefox or scan with MSE, Hitman Pro or other anti-malware capable program. Ads Not by this site is not that harmless, as it collects both info about sites you visit and displays advertisements.

What you need to do is remove an extension called Service x86. There are of course more such extensions, a short list is available here:

http://deletemalware.blogspot.com/2013/01/remove-ads-not-by-this-site.html

However, I'm pretty sure it's the Service x86 because I didn't see any other extension than this in a past few days. This one is being distributed rather actively. By the way, they all use the same pattern so maybe it would be a good idea for FF to block the whole "family" of these extensions.

And one more thing, if you won't remove the culprit, I mean adware that installed this extension, it will probably come back even if you reset FF. You may use a free tool called AdwCleaner to remove adware from your computer. Of course, you can use any other if you wish.

Try this working tutorial working tutorial.

or this working video