Talkback is a client application and server (plus server infrastructure and development/administration people) contributed to mozilla.org by Netscape. mozilla.org, many years ago, agreed to make an exception and include this product with our binary nightly and milestone distributions even though it's not open source because it provides huge value in debugging and isolating stability issues. Talkback has been used to identify and debug thousands of major crash bugs in Mozilla over the years and Mozilla is very happy to be able to include it in the Firefox testing builds.
How it works: A Talkback binary is packaged up with the Firefox browser binary. When the browser crashes, the Talkback application is triggered and it offers the user the option to participate. If a user says no then nothing happens. If a user agrees to help the Mozilla effort by submitting crash data then she is prompted with optional fields for including her e-mail address, the URL that triggered the crash and a comment. That user-entered data along with a stacktrace of the crash is sent to a Talkback server at Netscape which is accessible to many of the Mozilla developers. In aggregate, all of the crash data can very quickly point out specific problems being encountered by large groups of users. A small team of engineers pour through these aggregate reports and turn them into bugzilla bugs with good debug information which leads to quick fixing of the most high-profile stability problems. To see some of these bugs, query bugzilla for the keywords topcrash and topcrash+
What else: Talkback is not spyware, adware or anything of the like. Users are clearly prompted and asked to submit the report. User data unrelated to the Mozilla crash isn't at all useful to us. We only care about making Mozilla more stable. If you don't want to help Mozilla and Firefox become more stable by submitting your crash reports then don't. No data is being sent without your explicit consent. I'd encourage anyone that wants to see this browser improve to submit those reports. They are very, very helpful. But, like I said, if you don't want to, then don't. Just remember that we can't fix the bugs we can't identify. If you're happy seeing the same crash over and over then don't worry about sending in that report.
So, the next time you see a Firefox Talkback dialogue, submit that debug information!