It sometimes seems it's difficult to make money from making tools, because developers expect them for free, and with source (like Firebug). But of course it is possible, with products like Joel's FogBugz, like Jira and some IDE's.
Even serious money: Microsoft's Developer Tools (Visual Studio) business made over $1B revenue last year: http://www.techflash.com/seattle/2010/07/microsofts_11_billi...