That being said, there are still a few places in Firebug 2.0 where system colors are used. These may be replaced by self-defined colors in the future.
But note that Firebug is completely customizable via CSS and Jan 'Honza' Odvarko describes in his blog how to do that. So you can apply your own theme to Firebug using system colors or dark colors.
If you want to help to create a theme, which has better support for system colors, you should have a look at the code of the next Firebug version and create a patch for it.
Sebastian
Thinking about it a bit more, I suppose there are two types of widgets to consider. First are panels and buttons and such. Seems like these should follow OS settings just as Firefox does. Can't think of a good reason not to.
Next, there are the areas where syntax highlighting occurs, which as you mention become problematic. Unfortunately we are slammed here with work so learning how and writing themes for every plugin to a tool we use is not a good use of time.
So, I'm trying to think up a "quick" way to mitigate the problem, and these came to mind.
1. Have a menuitem that selects either light or dark value foreground colors for highlighting which can be manually toggled to the one that gives the best contrast.
3. Have a menuitem/checkbox to "just use the OS theme," unselected by default, but let's the user choose their own destiny.