Mozilla Build Lab was not that difficult.
I tried to build Firefox for my XP machine at home.
First, I was prepared for lots of problems. Only one problem I've got is that I did not follow instructions, so I forgot to make the installer.
Watching the movie by Mozilla's J. Paul Reed and following the instructions helped me to correct the error.
Second, I was prepared for a long build and posting my build log in wiki. The build took about 3 hours and I've got a pretty average machine [details available by request]. When I checked the build output in order to post it, I found it's about 8Mb+ big, so it's available by request too. (Otherwise it's going to be a good load test for wiki)
So here is my advise -- if nothing works -- RTFM
--Felis 22:40, 14 September 2008 (UTC)