"configure" is just the usual name of a script that creates a makefile. The
"./" tells the shell to look in the current working directory for the script
instead of $PATH. Some programs don't do this, perhaps instead using a script
that handles everything (The Ur-Quan Masters does this).
I'm not familiar with Firefox or Thunderbird's distributions (never tried
compiling them), but it might be this. See if you can find an executable file
(probably shell script, .sh) with a name like "install" or "build", and run
that.
As an aside, I don't see much point in Firefox over Abrowser. I suppose you
can get the latest cutting-edge release, but other than that, it's mostly
going to be identical to Abrowser.
