Mozilla firefox has certificate error issue.... please help!

Hello,

It's been a while I'm using mozilla firefox 27.0 beta without having a problem. but yesterday while I was surfing, a file got automatically downloaded and installed which I didn't care about.. but after that whenever i open firefox I got 'certified toolbar search' as my homepage and search engine as well. well I changed it to google.com so it's not the actual issue I'm concerned of.

But whenever I'm trying to open google, facebook, youtube etc. firefox showing that this site is untrusted... and if I get certificate and confirm security confirmation then the site opens but without an image........ well that is completely unusable. I uninstalled firefox 27.0 and downloaded firefox 29.0 but having the same problem.... even internet explorer and opera browser having the same problem for facebook, youtube etc..........

please help as soon as possible........

please please..