LMDE 2 support ends in December 2018, but LMDE bases on Debian Jessie, and Jessie’s support ends June 2020, so the only problem with LMDE 2 is Firefox, which loses updates after 2018.
The easy fix is to remove Firefox and install Firefox ESR. Via Terminal it goes:
apt remove firefox && apt install firefox-esr
… and if needed some language pack like Finnish
apt install firefox-esr-l10n-fi
If debian security repo is not on, it can be added using the next command:
echo '# jessie security
deb http://security.debian.org/ jessie/updates main' | sudo tee --append /etc/apt/sources.list.d/jessie-security.list