On the Development server, when using the ChannelAPI, it essentially degrades into a polling implementation instead of using Comet/long-polling. Thus, in your debugger, you see an endless stream of HTTP requests made to the server to continuously and methodically check for updates.
In essence, these are just AJAX requests, or as Firebug would like to think of them, XMLHttpRequests.
Since your browser is responsible for making these requests, the only way to disable them is to click the small arrow on "Console" in Firebug and uncheck the option for logging XMLHttpRequests.
Of course, this also disables logging for all of your other XMLHttpRequests. But it's a small price to pay for the clarity and serenity of a quiet, well-behaved JavaScript console.
For more helpful information on how to make the most of Firebug, see Firebug Tips and Tricks.
NOTE: This works for both users of the Python SDK as well as the Java SDK. (or Go SDK, assuming it has an equivalent ChannelAPI). This is not limited to only Python Appengine.
UPDATE:
From getFirebug:
Creates a time stamp, which can be used together with HTTP traffic timing to measure when a certain piece of code was executed.
The console.timeStamp method was released in Firebug 1.8.0. The same technique described above can also override this Firebug logging method.
console.timeStamp("This is the type of console logging statement that Google is using!");
The above logging statement would produce the olive text. This method can be disabled using the same techniques which were described in the previous section.
However, Google loads the console object inside of a closure, which means that, once Google's code is initialized, the ChannelAPI object has it's own copy of the console object.
In order to disable console.timeStamp, one would need to disable it as the very first action before anything else loads or runs. in other words, we would need to ensure that Google only gets its hands on the disabled console.timeStamp method.
For best results, load this code above the /_ah/channel/jsapi script tag to ensure the console.timeStamp method is disabled before jsapi loads:
if(window.console) console.timeStamp = function(t) { };
NOTE: Because Google invokes Firebug logging in this manner, the only solution may very well in fact require a bug report or feature request that would allow for programmatically disabling this level of logging. Alternatively, the Firebug team could provide a new version of Firebug that includes the ability to explicitly disable timeStamp log statements, similar to how they've done so with Errors, Warnings, XMLHttpRequests, and other log levels.