can we write test cases directly on Bugzilla?

Answer 1

No. You cannot write test cases directly into bugzilla.

Bugzilla is a BUG TRACKING tool.

There is a variant of bugzilla called as "Bugzilla Test Runner" which is a test management tool that uses the same database as that of bugzilla to efficiently interface with defect tracking. More information about BUGZILLA TEST RUNNER can be found at http://www.bughost.org/bugzilla/tr_testrunner/doc/short_using.html

All the best and let me know if you have more questions.