If you switch to the APT version, you will lose the bookmarks in the snap version unless you sync them with your Firefox account, or export them manually.
The other answer by Organic Marble is for Firefox-ESR, and the answer by eddygeek is for the beta version.
This answer is for the latest stable version of Firefox. You can use the Firefox PPA maintained by Mozilla team.
sudo add-apt-repository ppa:mozillateam/ppa
Then, copy and paste the following code in a terminal in one go (don't copy-paste line by line) to prioritize the apt version of firefox over the snap version.
echo '
Package: *
Pin: release o=LP-PPA-mozillateam
Pin-Priority: 1001
Package: firefox
Pin: version 1:1snap1-0ubuntu2
Pin-Priority: -1
' | sudo tee /etc/apt/preferences.d/mozilla-firefox
Next, remove the snap version of firefox
sudo snap remove firefox
If you see the following error,
error: cannot perform the following tasks:
- Remove data for snap "firefox" (1943) (unlinkat /var/snap/firefox/common/host-hunspell/en_ZA.dic: read-only file system)
Then run the following commands (source) to disable the hunspell service, and try removing Firefox snap once again.
sudo systemctl stop var-snap-firefox-common-host\\x2dhunspell.mount
sudo systemctl disable var-snap-firefox-common-host\\x2dhunspell.mount
sudo snap remove firefox
Install Firefox with apt.
sudo apt install firefox
To ensure that unattended upgrades do not reinstall the snap version of Firefox, enter the following command. Alternatively, you can turn off unattended upgrades.
echo 'Unattended-Upgrade::Allowed-Origins:: "LP-PPA-mozillateam:${distro_codename}";' | sudo tee /etc/apt/apt.conf.d/51unattended-upgrades-firefox
To undo these changes
- Remove the firefox PPA.
sudo add-apt-repository -r ppa:mozillateam/ppa
- Remove the apt pin.
sudo rm -rf /etc/apt/preferences.d/mozilla-firefox
- Remove the apt version and reinstall snap.
sudo apt remove firefox && sudo snap install firefox
Source: OMG Ubuntu