I have noticed that there are a lot of Firefox extensions out there for integrating del.icio.us with Firefox - some of them good (like Another Delicious Sidebar), and others not so much (like VeryDelicious, which is reportedly riddled with problems, as the user comments on the Mozilla Addons page for this extension indicate).
A common task is getting a list of bookmarks tagged with a particular tag name or a combination of tag names. A Firefox extension or even an XULRunner app was considered, where there would be a text box for the user to enter a tag (e.g., "sanity") or a combination of tags (e.g., "sanity+chocolate"), and a list of del.icio.us bookmarks with the given tag(s) then appears in a list box below it. A tag browser, basically. And then there are some nice-to-have features, like jumping directly to the URL or being able to edit a bookmark - add/remove tags, change the title and URL, delete it, etc.
But then I realized that a simple browser custom keyword can do the trick. Simply create a new bookmark with the URL of the form:
So to list all bookmarks tagged with "sanity", you would type in the browser's location bar:
tag sanityAnd to list all bookmarks tagged with both "sanity" and "chocolate":
tag sanity+chocolateWith such a simple solution that most adept users can implement, it's laughable that one would create an extension (or hell, a complete XULRunner app) that would do the same thing. The custom keywords feature found in Firefox, SeaMonkey, and Mozilla 1.x is sufficient enough for listing all your del.icio.us bookmarks tagged with a particular tag or tags; and the del.icio.us site itself works very well for managing your tags and bookmarks. It's all about keeping it simple - and leveraging existing solutions to solve new problems.
Update: Apparently, there is a del.icio.us search engine addon for Firefox.