It looks like the functionality to edit a cookie's value has broken with the latest Firefox update. I'm running Firefox 46.0.1 and Firebug 2.0.16 on OSX 10.9.5. Here are the steps to reproduce:
1. On the cookies tab, right click a cookie and click 'Edit'
2. The window that opens will now be too small, so you will need to expand it to the right
3. Change the cookie's value
3. When you click 'OK', the cookie disappears from the cookies tab
4. When you reload the page, the old cookie is re-generated by the page
Thanks.