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.
The tinderbox webtool also reports the results of various performance tests such as Tp or Txul. Mozilla developers frequently check these numbers to observe the effect of code changes on performance; a change that sufficiently slows performance may be backed out.