How can I clear individual cookies on firefox?
Firefox for Android stores Cookies  in /data/data/org.mozilla.firefox/files/mozilla/{YOUR_PROFILE}/cookies.sqlite. 

You can't peek into Firefox's data directory without having elevated privileges than what is granted to a normal user, call it superuser access. If you have root access then simply pull that file into PC and open it in an SQLite viewer. Search the Cookies using domain name and purge the ones bothering you. Save the changes and push the file back to its location. When doing that, make sure to delete the files cookies.sqlite-shm and cookies.sqlite-wal in the data directory of Firefox. They are temporary files created when cookies.sqlite  is opened.

For a non-rooted device, you may consider pulling the data directory using adb backup. Extract the backup using ABE. Make the changes, pack the directory into a backup file and restore it in the device. I don't guarantee that restore would work.

There are some addons here which you may try and tell us if they work. They should work regardless of superuser access.