The following links may prove very useful to you as you develop Firefox.
http://lxr.mozilla.org/seamonkey/
A browse-able version of the CVS repository, showing the current state of all source files and revision histories.
Right click on each of the text boxes on this page and choose "add keyword for search", and create keywords for each. This will be very useful to you. Create a keyword for lucky searches, e.g. "lf". When you type "lf foo" you will go to the first file name returned for that search.
https://bugzilla.mozilla.org/query.cgi
An advanced search form for the bugzilla database. Familiarize yourself with the content of this page, and use it to produce lists of bugs you're working on or are tracking. You can then subscribe to your favorite bug lists using a RSS reader like Live Bookmarks.
A query system for changes made to the CVS repository. You can look up changes to a certain set of files or directories, on any branch, by any user, in any time period. Useful for bug sleuthing.
http://tinderbox.mozilla.org/Firefox/
If you get commit access to Mozilla, you will come to know this page well. Even if you don't, if you pull source from CVS you will want to check this page before you do so, because it will tell you if the tree is in a good state.
http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/
You can find nightly Firefox builds for testing here. For old builds, you can look on http://archive.mozilla.org/