As I understand it, the idea behind the "thin blue line" flag is that there's a "thin line" of police between society / civilization and chaos (which they presume would exist / rule in the absence of coercive force / threat from armed agents of the state). I'm not really sure what the thin red line is supposed to represent, other than maybe an attempt to associate firefighters with police. So, if the goal is to express a need for mental health support for or love of firefighters, I don't think a "thin red line" flag is conveying that meaning to me. Something more direct might better convey that message, if that's what you intend.