Some users have noted that when they have Chrome browser installed, it becomes the viewer used when clicking a link in Firestorm, even if you have already set another browser as the default. The reason for this is because of some settings that appear to not apply globally
From this jira comment:
On a debian- or ubuntu-flavored distro, I typically need to run both of these to get all programs to call the desired browser.
sudo update-alternatives --config gnome-www-browser sudo update-alternatives --config x-www-browser
Looking at the Linux start scripts for common SL viewers, x-www-browser is the first one tried, so that's what you want to tweak.