Comment 17 for bug 2043820

you must remove both deb packages and snaps otherwise it will cause issues

before upgrade do this :

sudo apt purge firefox firefox-*
sudo snap remove firefox

upgrade to 24.04
after upgrade add this mozilla respository

https://launchpad.net/~mozillateam/+archive/ubuntu/ppa

sudo apt install firefox firefox-locale-en

this should bypass snap instalation and allow you to upgrade