The new Firefox 30.0 has been released along with Firebug 2.0. Unfortunately, some of you may experience Firebug tool not working. The issue is caused by 'lazy oval optimization' via in Firefox.
The solution is do one of the 3 following options:
1. Revert Firefox back to previous version.
2. Upgrade Firefox to 31.0 ALPHA
3. (my preferred choice) Uninstall Firebug 2.0; close and restart Firefox. Go here to download the previous version of Firebug (1.12.8 - down near the bottom of the page). Install it via using Firefox add-on installer.
If you have auto updates turned on, you may want to disable that temporarily until the new Firefox 31.0 comes out or else Firebug may update the 1.12.8 version to 2.0
Edited by Eddie, 19 June 2014 - 09:20 AM.