Try following the instructions in my tutorial 'How to Import & Export Favorites in IE7'. Although this tutorial concentrates on the favorites, you can use the same procedure for cookies (and feeds, if you so wish). Cookies are backed up into a text file, and you can edit that file as normal using Notepad. The edited file can then be re-imported, again following the instructions in my tutorial. There should be a similar function to import & export in Firefox and other browsers, so you might like to try the appropriate function for your browser.
Dwarf
How to Import & Export Favorites in IE7