I'm a big fan of using search engines but I want to use the @ and # symbols to find users and hashtags easily. Basically, when using a search engine, e.g. Twitter, when you use @ at the beginning of your search, you're opting to be directed or search for a user profile. The same goes for # where you're opting to be directed or search for a tag or hashtag.
Facebook and Instagram are other examples that uses @ and #, and I would like Firefox to add these as optional reserved keywords for developers to use. Even if a website doesn't have users or hashtags, that's fine, its up to developers to decide how @ and # work for their engines, or choose not to use them at all. This shouldn't affect the way search engines work (apart from the search shortcuts) but compliment how the Address Bar works as well.