I used to have this extension installed a while ago and it was pretty cool but I got rid of it because it slowed down Firefox a lot on some websites.
This version introduces crash protection so this shouldn't happen any more. Among the new stuff in version 0.3:
- "View Source Live"
- Computed style inspector
- Events inspector
- Quick search filtering
- Crash protection against sites with a bazillion errors
- DOM editing
Also at Mozilla Update.