Linux Mint has issue with some default apps that are launched from Firefox and Chrome browsers. For example instead of PDF viewer GIMP is lauched as PDF viewer. After investigating this issue looks like it is a common issue for lots of people who are using Mate version of Linux Mint. Probably some updates are to blame.
As usual best place to find good info is Arch Wiki which has some great info about setting default app launchers.
Issues for me was opening PDF files and directories from Firefox and Chrome. To check current default apps 'xdg-mime' is used:
xdg-mime query default inode/directory
xdg-mime query default application/pdf
and a quick fix for my two issues was:
xdg-mime default atril.desktop application/pdf
xdg-mime default caja.desktop inode/directory
and now just to test if new launchers work as you expected:
ps. Ask Fedora has really informative page regarding default apps on Fedora.