I installed the firefox dev edition by extracting the tar file from https://www.mozilla.org/en-US/firefox/developer/ at /opt/firefox_dev_edition.

But when an update was available, firefox showed me this error:

Update available
Firefox couldn't update automatically. Download the new version - you won't lose saved information or customizations.

Turns out the /opt/firefox_dev_edition directory was owned by root user, which resulted in Permission denied error when firefox tried to update itself. So, I changed the owner to the main user, which I use the system through:

chown -R "$USER":"$USER" /opt/firefox_dev_edition

This command changes both the user and the group of the directory and all of its files and sub directories recursively(the -R option is an alias for --recursive)