Google add-on search: A lot of the time when I click on the search results, it sends me to a site called "Infomash"!?

Question

Don't know what I've done wrong, all I can think of is update firefox in the past few months. I use Malwarebytes and bitdefender. I also have Stinger installed. Nothing is picking up a hijacker or anything. However, everyday...when I use firefox (only browser I use) I do a search through the google add on, and the results show up. I click on the one I want, and about 1 out of 5 times a site called "infomash" pops up. I hit back, click on the same result...and I get what I wanted. NOTHING will detect this!
All of a sudden tonight, I've been searching things for a video game. Looking where to find a gun for the game. CLEARLY state the name of the game and everything in the google search.
I click on this link: (Not actual link, but search result above it)
http://fallout.wikia.com/wiki/Xuanlong_assault_rifle
And it takes me to this link!
http://www.brownells.com/
I hit the back button, click on the link search result above again....and that time it takes me to the right one.
I don't play games on this pc, don't have several things installed. Mainly just use it for email (don't download hardly anything from email), facebook, amazon, ebay, and work.

Answer 1

This is indeed caused by a virus - and that I'm 99.99% sure (beware of the 0.01% though) - I've searched some forums around and it really seems to be malware.
Edit: Be sure to update your current paid antivirus and to perform an on-demand scan before buying another.
You're gonna need other security tools to test this, since your infection is probably TDSS or others. These kinds of malware tend to install more malware, that installs more malware, and so on.
Your best bet here is a paid antivirus, but there are more antivirus in this world that you haven't tried, such as,
[http://www.safer-networking.org ],
[http://www.spywareterminator.com] ,
[http://www.microsoft.com/security/malwareremove/default.aspx]
If none of these work, I'd highly recommend a paid antivirus (this is a potential rootkit, not exactly the easiest thing to detect)
Good antivirus to choose from:
-TrustPort*
-GDATA*
-Kaspersky
-AVIRA
-F-Secure -ESET
-BitDefender (the one you own) -eScan
Antiviruses marked with * are partially based on BitDefender, and it might be best to look into others.
(in order of my personal recommendation in terms of detection-it might not be worth anything and it doesnt account for speed of scanning)
There are some free tools that have been "claimed" to solve this issue.
BEWARE: THESE TOOLS ARE A MERE PART OF RESEARCH AND ARE NOT ENDORSED BY MOZILLA. USE AT YOUR OWN RISK.
[http://support.kaspersky.com/viruses/solutions?qid=208280684]
[http://forums.majorgeeks.com/showthread.php?t=182559]
That took long to write!
Thanks for asking.