The Fedora developers want to hear about every specific and reproducible bug that happens when you use Fedora, but it does not help to have the same bug reported many times. You can help to get more bugs fixed faster by checking the Bugzilla database for duplicate reports, and marking a duplicate report if the bug you found has been reported already.
If the bug seems obvious, or you found references to it on a major site, check the most frequently reported bugs list .
Using the Bugzilla query interface
The Bugzilla query form looks dauntingly complex - it is complex and powerful—but you can safely ignore most of the form. Any part of the form that is left blank does not limit the search at all, and each part that is filled in cuts the list of bugs down to only those that match the criteria you set.
The Status field is set by default to find NEW, ASSIGNED, NEEDINFO, and MODIFIED bugs—the unfixed bugs (or in the case of MODIFIED the recently fixed bugs).
In the product filed. you should always use "Fedora". In the component field. select the SRPM that contains the defect. See here for more details.
Enter a word or short phrase that identifies the problem you saw as uniquely as possible in the Summary field. Examples: view source, auto proxy, drag drop, png image. If you enter more than one word and they are not a phrase, change the type of matching for the Summary field from contains the string to contains all of the words/strings or contains any of the words/strings, as appropriate (just to the left of the text-entry field).
Don't use the Keywords field without first clicking on the word Keywords: , you can't type just anything in there. If you try to use this for keywords that are not on the list, it won't work. Fedora generally doesn't make use of keywords in it's use of Bugzilla, however for things like enhancement requests, FutureFeature may be used, or for tracking bugs, the Tracking keyword is used.
Mozilla has a nice tutorial on using the various text search options, including the boolean charts at the bottom of the page.
What to do with duplicates once found
When you find a duplicate bug, it is tempting to just mark the higher-numbered one a duplicate of the lower-numbered one. Resist the temptation to do this, instead, close the one that has less information. This may very well mean closing a lower-numbered bug and keeping a higher-numbered one open - that's fine.