The Mozilla source tree is here.  It is (manually) kept updated with
the Mozilla CVS repository.

I log when I "pulled the tree" (updated with CVS update) in the file
WHEN-LAST-UPDATED.  The last line in WHEN-LAST-UPDATED is when I last
updated.

Also, you can browse the Mozilla source on the web at
http//lxr.mozilla.org/
It is very convenient as it allows you to search the source.