From the Bugzilla main page (http://bugzilla.mozilla.org), choose "Enter a new bug".

Select the product that you've found a bug in.

Now, fill out the form. Here's what it all means:

Where did you find the bug? --- which product,which product version,which component, which hardware platform ,which Operating System

How important is the bug? -- Severity

Who will be following up on the bug? -- Which engineer should be responsible for fixing this bug? Who else should receive e-mail updates on changes to this bug?

What else can you tell the engineer about the bug? -- On what URL did you discover this bug? How would you describe the bug, in approximately 60 or fewer characters?