Bugzilla can be a bit intimidating when you use it for the first time, but we have documentation to help you make useful bug reports here that may get you started:
[https://developer.mozilla.org/vi/docs/Mozilla/QA/Bug_writing_guidelines Bugzilla: Bug writing guidelines]
Don't hesitate to ask experienced Nightly users (staff or employees) to help you file the bug in the #nightly IRC channel if you are unsure about the process or if English is not your native language and want to make sure your bug description is understandable.