Does it happen just for your search engine links or for every link you click on (including inside the websites)? 9/10 times such redirections are done by some toolbars/plugins installed by outside programs (e.g you installed a freeware program and agreed to installing a new toolbar/search engine to your firefox). Try looking for these. If you find them, just uninstall them. Also, search for malware. If that doesn't help, try making a screenshot of the changed links so that we'll know what you're dealing with.