If you've installed Hiri as a snap package on Linux, you might not be able to open links in your emails. This is a problem with snaps which can be resolved by installing an additional package:
sudo apt-get install snapd-xdg-open
This package enables applications to use xdg-open to open links in the browser. There is an effort to merge snapd-xdg-open into snapd so in the future it'll just work.
Unfortunately, this package might not be available on your distro if you're not running an Ubuntu-based one.
In that case, it's best that you download and use the binary tarball.