The icon of the wonderful plugin Firebug recently moved from its familiar location at the bottom right corner of Firefox. This was due to a UI change in the status bar of Firebox from version 4 onwards. The Firebug blog does a good explanation of why this was necessary.
What I did to fix however was to type about:config in my Firefox address bar.
I accepted the “I’ll be careful, promise!” warning.
To narrow down the (many) configuration options I applied a filter “firebug.show”
Finally I changed “extensions.firebug.showStatusIcon” to true.
29/01/2012 From the number of pageviews this post receives I see this is common problem. I hope it helps people solve the issue.