Lots of people are unable to use the Style tab in the new Firebug beta. The Firebug FAQ leads to a thread suggesting this fix:

1. Uninstall Firefox
2. Delete the (left-over) install folder (e.g. C:\Program Files\Mozilla Firefox)
3. Reinstall Firefox using a custom install (to ensure the DOM Inspector is installed

Knowing that I already had the DOM Inspector installed, I thought I’d try removing just a few files, instead of the whole install folder. Whatever I did worked, and the Style tab works for me now. This more conservative procedure is:

1. Uninstall Firefox
2. Delete only the named inspector* in C:\Program Files\Mozilla Firefox\components
3. Reinstall Firefox using a custom install (to ensure the DOM Inspector is installed

I’m not sure whether the uninstall and reinstall procedure is strictly necessary, so I’m hoping someone out there can try this:

1. Stop Firefox
2. Delete all files named inspector* in C:\Program Files\Mozilla Firefox\components
3. Restart Firefox