Updates for your web browser Firefox come from Mint's own repository, not straight from the Ubuntu sources. In some cases, this might mean a delay of several days before you get the latest Firefox.
If you don't want to wait, you can instruct your Mint to prefer the Ubuntu version of Firefox. Like this:
a. Launch a terminal window. (You can launch a terminal window like this: *Click*)
b. Then create a certain new file by means of this terminal command (use copy/paste to avoid typing errors):
sudo touch /etc/apt/preferences.d/firefox-ubuntu
Press Enter. Type your password when prompted. In Ubuntu this remains entirely invisible, not even dots will show when you type it, that's normal. In Mint this has changed: you'll see asterisks when you type. Press Enter again.
Then copy/paste this line into the terminal, in order to edit the new file:
c. In that empty text file, copy/paste the following blue text:
Pin: release o=Ubuntu
d. Save the modified text file and close it.
e. Launch Update Manager, refresh it and install the updates for Firefox (if any).
f. Close Firefox and launch it again.