Tools > clear private data > check everything
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try deleting your private data under tools
just use ccleaner....
First open ccleaner
internet explorer.
Check the ''Autocomplete Form History''
you can download ccleaner from here
www.ccleaner.com
Certainly follow the instructions from Mintiecookie they're spot on but I would also run an anti-spyware scan and anti-virus as well. If you've not used Firefox after re-downloading and there are websites showing then you might have something in your system. Its worth running them either way
Click Tools on top task bar then down to the 3rd last option 'Clear Private Data' this will erase previous browsing history, saved passwords, cookies which is all optional as you can choose what you want to do.