Well if your formhistory resets at closing firefox then try reinstalling firefox (fixes most of annoyences for me when firefox acts wierd). when you type about:config in your adress bar in firefox and in filter enter privacy at bottom there should be sanitize on Shutdown make sure it is not set on true. If it resets after restart of windows it most likely windows fault.

For full Firefox reintall backup bookmarks in C:\Documents and Settings\*********\Application Data\Mozilla\Firefox\Profiles\wiroeqw3.default

and delete profile then install fresh firefox and copy back bookmarks and install addon back.

But that little extreme since it minor annoyance and try just install addon that deals with forms.