Update: LiveHTTPHeaders 0.17 has been released, so this hack is no longer needed or recommended.
LiveHTTPHeaders, one of my favorite Firefox plugins, hasn’t been officially updated for Firefox 4.0 compatibility yet. I hacked the .xpi file so it will install in FF4. If you’re impatient like me, grab it here. It’s working fine for me, but I can’t guarantee it will be 100% stable.