As I was stating in an earlier post about new software, particularly automatic software doing stuff you don't want, seems the 'duplicate removal / clean up' managed to delete the repository for seamonkey... needless to say it might be missing and not updating seamonkey for you like it should....

opening a terminal and entering the follwing below should do it, if it doesn't follow the link for manual instructions

echo -e "\ndeb http://downloads.sourceforge.net/project/ubuntuzilla/mozilla/apt all main" | sudo tee -a /etc/apt/sources.list > /dev/null
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More detailed instructions are available here.

https://sourceforge.net/p/ubuntuzilla/wiki/Main_Page/#installation