Firefox 3.0 and earlier used CVS as its version control system, and if you were still using cygwin tools then you could choose which line endings you wanted to use, so for instance you could use the MS-DOS editor. However by then most people were by then using MSYS (conveniently distributed in a mozillabuild package that contained other components necessary to build) which only supported Unix line endings. This was not a problem since it was supplied with Vim (5.8.9 I think) which was configured to default to Unix line endings but would also open files with DOS line endings although it would warn you that you were using the "wrong" line endings. Of course if like me you're also used to using Vim on Unix then MSYS Vim looks much the same.
Now that Firefox has switched to Mercurial, all the source files always have Unix line endings even when checked out using a Windows version of Mercurial.
Now the latest mozillabuild package does not ship with the MSYS version of Vim, instead it ships with Vim for Windows (7.2) which doesn't really integrate well with a Unix toolchain. I know I can
set fileformat=unix to make Vim default to creating files with Unix line endings (this also affects Firefox developers; from time to time I see new files checked in with DOS line endings because that's how Vim for Windows creates them by default), but is it possible to make it warn me when I open a file with DOS line endings? Currently it warns me whenever I open a file with Unix line endings, which is all the time. Example:
"~\.vimrc" [unix] 9L, 155C