i don't think you can manually edit the cookies.txt file without changing its structure and probably corrupting it. since the cookie is intact in firefox.. try this.
first install this extension in kmeleon
then install this extension in firefox
it's the same extension but with modified install.rdf so you can install it in ff 3.x and above without firefox complaining about incompatibility issues
restart both browsers after installing the extensions
in firefox click on the tools>cookie editor to open it
in kmeleon click on tools>privacy>cookies manager to open it
in the addnedit cookies dialogue in firefox, select the cookie cl_def_hp and click the edit button
this will open a second dialog with all the bits of info of the cookie.
in kmeleon in the addned cookies, click on the add button
this will open a second dialog to enter the custom cookie info.
enter the exact same values from firefox edit cookie to km add cookie (copy and paste) and click the save button. the new cookie will now be displayed in the main dialogue.. browse to craiglist and test
see this snapshot
hopefully it work, i tested on youtube where i wasn't loggedin in kmeleon and then logged in in firefox and after iadded the cookie i refreshed the page and youtube recognised it and showed me as signed in