I have not used this before, but with a little research it looks like it competes with Firebug? http://sourceforge.net/p/krumo/bugs/11/ If you disable this extension does it work as normal?

See also http://ratatosk.net/drupal/tutorials/debugging-drupal.html

Thanks for the help, guigs2. I had seen that bug report also. I tried it with Firebug disabled, but it didn't help. This used to work just fine running alongside Firebug. It seemed to stop working after Firefox updated itself in the last few months, but I wasn't working on this development project for a while. The same page works in Chrome and IE9, so it is likely a Firefox issue. This renders FF almost useless to me, where it used to be my favorite browser. I'd love to get it sorted out.

Well, a little egg on my face never hurt too bad. I did some more checking, and changed my site to a different Drupal theme. The krumo display works OK. It must be something in my custom theme. At least I know where to look now. Thanks for the help. Sorry for the unecessary post.

HI ssalive, Thank you very much for posting back! It may be useful to post a bug about that theme in the http://dev.webcompat.com