For reasons we are not sure, in your updates from Canonical you may not realise that the firefox installed is suddenly uninstalled, with the firefox icon disappearing from the Plank dock.
EDIT: Canonical now acknowledge the issue which you can subscribe to https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/1948482
In its place - via the menu is firefox … but when you run it for the first time it will take much longer to start … and then it will look very different from before.
It is kind of too late to resolve this boo-boo in the updates from Canonical. Oh dear.
If you want to swap back to the deb version of firefox then:
snap remove firefox
sudo apt install firefox
If you want to stick to the snap version of firefox but you want it Pocillo themed:
snap install gtk-theme-pocillo
for PLUG in $(snap connections | grep gtk-common-themes:gtk-3-themes | awk '{print $2}'); do sudo snap connect ${PLUG} gtk-theme-pocillo:gtk-3-themes; done
for PLUG in $(snap connections | grep gtk-common-themes:gtk-2-themes | awk '{print $2}'); do sudo snap connect ${PLUG} gtk-theme-pocillo:gtk-2-themes; done
If you want to stick to the snap version of firefox but you want it QogirBudgie themed:
snap install gtk-theme-qogirbudgie
for PLUG in $(snap connections | grep gtk-common-themes:gtk-3-themes | awk '{print $2}'); do sudo snap connect ${PLUG} gtk-theme-qogirbudgie:gtk-3-themes; done
for PLUG in $(snap connections | grep gtk-common-themes:gtk-2-themes | awk '{print $2}'); do sudo snap connect ${PLUG} gtk-theme-qogirbudgie:gtk-2-themes; done
There maybe other themes available that can be similarly installed and themed - do look via Snap search results for 'theme' — Linux software in the Snap Store
For all that being said - remember at 22.04 you will only be able to install the snap version of firefox - the deb will be forcibly removed. The reasons for this are covered here: Feature Freeze Exception: Seeding the official Firefox snap in Ubuntu Desktop - Announcements - Ubuntu Community Hub