One of my favorite features of Mozilla, for several years now, has been custom keywords which allows users to name bookmarks for faster access. I, for example, have named my bookmark for mozillaZine.org "mz" and typing "mz" into the address field takes me to that bookmark location. A second, and lesser known, feature of of Mozilla's bookmark handling is the %s wildcard flag. Users can use %s as a variable in URLs and when combined with custom keywords it becomes a very powerful tool.
A good example of this is for searching like at Bugzilla or Google. I have the bookmark http://bugzilla.mozilla.org/show_bug.cgi?id=%s which I've given the keyword "bug". Now typing something like "bug 12345" in the address field is interpreted like this: the "bug" keyword gets resolved as the bookmarked URL http://bugzilla.mozilla.org/show_bug.cgi?id=%s and then the %s variable gets replaced with "12345" yielding the new URL http://bugzilla.mozilla.org/show_bug.cgi?id=12345 which loads up the bug I was looking for. A similar example is my custom keyworded Google bookmark. The bookmark is http://www.google.com/search?q=%s&btnG=Google+Search and my custom keyword is "g". Typing "g foopy" does a Google search on the word "foopy".
Why do I bring all of this up again? Well, master of bookmarklets and all-around-great-guy, Jesse Ruderman has posted another batch of bookmarklets that combines this Mozilla custom keywords feature with another powerful Mozilla feature -- that the address field can interpret and execute arbitrary JavaScript and return results in the browser window. These new bookmarklets, along with scores of others available at Jesse's Squarefree website, should be extremely useful to JS and Web developers. Check it out and if you've got any creative uses of Mozilla features that you'd like to share let me know by clicking on the "Comments' link below.
update:I really should also point out that these features can go a lot further than what I've suggested here in this post. Check out grayrest's page on Mozilla bookmarks to see some more of this powerful feature.