It's been said before, but with the recent posts on cleric party list problems, I figured this could stand with some reiteration:
When health bars are over a background that is largely red/orange, such as while standing in a firewall, or in a chamber like the Fernia chamber in the Prison of the Planes, it is very hard to see the health conditions of party members.
This could probably be rectified with an alternating background color immediately behind, and a pixel or two larger, than the red health-bar. It should provide enough contrast to detect differences in the health bar with the colors of a flaming background.