It will look for user specified cookies file and if found import it's content on browser's startup.
The new v0.3 added GUI options for Firefox 13 and newer accessible from your addon's list.
For outdated versions of Firefox (12 and older) you can change path to the file using about:config and the following setting:
In addition if setting extensions.autorestorecookiesonstartup.delete
set to true, it will delete all cookies before it import them from cookies.txt file.
If you create cookies.txt file with Cookies Manager+, make sure you use Backup feature, not Save as file