Firefox should make back ups of your bookmarks. As the back ups are hidden files you will first have to go to folder options to change the settings to show them. Click start > control panel > folder options > click on the "view" tab and place a tick beside "show hidden files and folders > apply > OK. Now go back to start > my computer . c drive > documents and settings > then choose the folder with your name > application data > mozilla > firefox > profiles > then a folder with a set of random numbers. You should now see a folder containing bookmark back ups. If you open that folder you will have two or three to choose from.
To import the bookmarks back in to firefox.....open firefox > on the toolbar click on bookmarks > organize bookmarks > file > import > place a tick in the "from a file" box > next. Now navigate to your bookmarks back up and double click the one you want to use.