I just read a blog post about how to ‘Block Firebug on your site‘. And I really think it is something you shouldn’t do, no matter if Firebug makes your web page completely unusable or not. What I think you should do is to make a suggestion.

<script type="text/javascript">
if( window.console && window.console.firebug )
    document.getElementsByTagName('body')[0].innerHTML += "<div onclick=\"this.style.display = 'none'\" style='position:absolute; top:0; width:100%; padding: 5px 0; background: #ff7; border-bottom: 1px solid #770; font-weight: bold; text-align: center;'>Firebug can make this web page slow, we suggest you disable it for this web page. Click to close this warning.</div>";
</script>

Adding the code snippet above to your web page will notify the user that ‘Firebug can make the current web page slow’ and make the suggestion that the user should disable it. Look at the image at the top of this post. Adding that yellow bar to the top of the page is all, I think, you should do.

This follows a line of thought I think you always should have when creating web pages. Rarely enforce or disable. Instead you should lead your visitors to the right path with feedback and suggestions. Like that they probably should use strong passwords and, if it slows down your web page, disable Firebug.