Recently my Firebug stopped working. I googled for any problems between my Firefox version 3.0.11 and Firebug 1.3.3, but I only found a few topics which didn't fix my problem. After a few attempts I realized there are a few reasons for why this might happen. My problem was the 3rd case.
Case 1 Incompatible FB version or other add on.
You need to check if some other add on is not interfering with FB. To check this you can create a new Firefox profile which will start a "fresh copy of Firefox". In this copy, install firebug and see if it's working. If it is, all you need to do is try disabling some of the add-ons from your main profile as those are most likely to be the root cause.
If this doesn't solve it, read further.
Case 2 Yahoo Toolbar or Skype Add-on
These 2 add-ons are installed in the background and might not show in the add-ons list. If you install Console 2 Firefox add-on, it should show a message like “Firebug is null” each time you try to start FB. You can find on the internet tutorials about how to remove them. The reason why these 2 aren’t covered in the first case is that they are “default add-ons” so they are copied to any new profile.
Case 3 Dot Net Assistant for Firefox
It seems Microsoft includes in the System Updates, an update for .Net Framework which includes a "secret Firefox add-on". This is never mentioned in the updated installation process, so as far as I'm concerned it's a piece of software installed without my will. This messes everything up. To remove it, use this tutorial : http://www.dedoimedo.com/computers/ms-dotnet-firefox.html . I used the second method, registry removal as it seems better. This problem shows the same message as in case 2, Firebug is null, but it also shows an error from dotnetassistant .