Nikhil Kothari, the author of many of the Microsoft ASP.Net controls, has written an extension for Internet Explorer to detect when the ASP.Net page is running in Debug mode or in Trace mode and alerts the developer by showing information in the IE sidebar.
There's just one problem - it only works in Internet Explorer. So, what's a bored developer to do? Why, create a version for Firefox!
Introducing the Firefox ASP.NET Development Helper.
UPDATE: Until I ask my sysadmin to allow .xpi as a valid IIS6 MIME type, download the XPI here: Firefox ASP.NET Development Helper. Make sure you save it as aspnetdevhelper.xpi, no matter what the download dialog says.
It tries to be as faithful as possible to Nikhil's original extension, but obviously the development model between a Firefox XUL extension and Nikhil's "winforms-embedded-in-a-browser-helper-object" combo can't be understated. In addition, there is no View State decoder in the Firefox implementation.
In order to test it out, you will need to download Nikhil's code and install his HttpModule into your ASP.Net v2 website. This allows the extension to detect debug and trace modes.
Finally, it's worth pointing out that this is completely open source! It is licensed under the CPL. You can get the entire codebase here.