Get a Bugzilla account
Mozilla's bug tracker is at https://bugzilla.mozilla.org/, which is often abbreviated as
You don't need an account if you simply want to build the code and modify it, but you will need an account in Bugzilla if you want to file or comment on bugs, send patches, get assigned to bugs (so you can 'claim' them), etc.
Note: if you are a Mozilla employee, don’t use an email alias to sign up, use your full LDAP account.
To make yourself easier to find by other colleagues (for example when they're trying to set a reviewer for a patch), you can edit the real name field to add your alias or any other word they might use to search for you there. The convention is to use something like
Your Name :alias :ldap/:ircnick. For example:
Mary Smith :mary :msmith