If port 6080 is already in use when I select Tools > Web Developer > Remote Debugger, nothing appears to happen, except that the following exception is reported to the Browser Console: uncaught exception: 2147746065 At the very least, Firefox should tell me about the problem; even better would be to resolve it automagically, f.e. by using a different port.

We have code to retry different port numbers here: http://mxr.mozilla.org/mozilla-central/source/toolkit/devtools/server/tests/unit/test_dbgsocket.js#122 But we are also planning to use a local domain socket for the Browser Debugger when those are implemented (maybe they already are?). I'm leaning towards the latter if this bug is not considered high priority.

Not specific to the browser debugger, but if the port is not available, then I think an alert should be shown if devtools.debugger.show-server-notifications is true.

Sorry, I didn't see bug 958498.

This should be fixed as part of bug 958498. Marking as duplicate.