When I scroll through through the comments of a bug I end at the bottom of the page. When I then want to search a new bug or a related one, I need to scroll all the way back to the top of the page.
A search box at the bottom will solve this and help usability.
Version: unspecified
Severity: enhancement
I normally use "End" and "Home" keys on the keyboard for that, plus there is a "Top of page" link at the bottom.
This feels sufficient enough so I don't favor duplicating certain functionality (except for stuff that's duplicated by default anyway, e.g. the "Bug List: First Last Prev Next" section) in Wikimedia Bugzilla (also because the interface feels a bit cluttered already anyway).
If you have a strong opinion please feel free to file an enhancement request upstream in bugzilla.mozilla.org against the "Bugzilla" product.