Firefox problem

Hey guys, recently my browser has been messed up. When I click a google link, it will take me to an add or something, then when click back and do it again, I go to the right site. When I click “Last Post” on any forums, it will take me to:
And not the real site.

Some sites I can just plainly not see giving me:
Bad Request

Your browser sent a request that this server could not understand."

Any help?