Have you looked at the bookmarks themselves to see if they have been altered from what you thought you were saving? If so, then you should be able to correct that, but check if you save a new bookmark whether or not it was also altered. If so, then you have a virus. If not, then your browser is seriously pwnd in that when navigating via the bookmark the URL is being dynamically altered. In any case, this is the first I have heard of this problem. :-(
A first for me, also. You could try Adwcleaner from http://www.bleepingcomputer.com/download/adwcleaner/
- run the exe, hit Scan. When it completes you can press Report and post the result here for advice. Review finds carefully if you wish to Clean unaided.
As an aside, Ghostery will block adcash and similar sites cold. Worth getting, esp cos it's free.
With FF, if you pause on the bookmark in the list you should see the URL pop - if then you are truly redirected something has infected FF, likely via an extension. Best to reinstall it, but make sure to delete the Mozilla folder under Applications in your profile first.