Tinderbox is a webtool that Mozilla developers use to check whether the current CVS code compiles and passes basic functional tests.
Mozilla runs build computers ("Tinderboxen") that continually build the latest source code; the tinderbox webtool shows the status of these builds. Use the tables there to find out the status of the source tree for the platform, product, and code branch you are interested in. If it is green, that means the latest CVS code compiles and passes the smoketests. If red, it fails, and your build likely will fail too.