when i use a search engine like google, yahoo, etc., when I click on the link I am immediately redirected to a different site. How to I prevent this?

its not firefox related issue,your pc is infected,ask your antivirus technical support to solve this kind a problem,its a malware called "redirect virus"

Install, update, and run these programs in this order. They are listed in order of efficacy.<br />'''''(Not all programs detect the same Malware, so you may need to run them all to solve your problem.)''''' <br />These programs are all free for personal use, but some have limited functionality in the "free mode" - but those are features you really don't need to find and remove the problem that you have.<br /> ''Note: If your Malware infection is bad enough and you are mis-directed to URL's other than what is posted, you may have to use a different PC to download these programs and use a USB stick to transfer them to the afflicted PC.'' Malwarebytes' Anti-Malware - [http://www.malwarebytes.org/mbam.php] <br /> SuperAntispyware - [http://www.superantispyware.com/] <br /> AdAware - [http://www.lavasoftusa.com/software/adaware/] <br /> Spybot Search & Destroy - [http://www.safer-networking.org/en/index.html] <br /> Windows Defender: Home Page - [http://www.microsoft.com/windows/products/winfamily/defender/default.mspx]<br /> Also, if you have a search engine re-direct problem, see this:<br /> http://deletemalware.blogspot.com/2010/02/remove-google-redirect-virus.html If these don't find it or can't clear it, post in one of these forums for specialized malware removal help: <br /> [http://www.spywarewarrior.com/index.php] <br /> [http://forum.aumha.org/] <br /> [http://www.spywareinfoforum.com/] <br /> [http://bleepingcomputer.com]