(In reply to Mike Hommey [:glandium] from comment #16)
I'll pile up with my own use case for which I filed bug 1678190 for: I have a keyword for this bugzilla. And while yes, technically, you could say that the page you get to is a search page because it has a search field, it also a large button for creating new bugs, and a bunch of other functionalities. I was able to either use "bz search string" to search for a specific bug or "bz" to get to the bugzilla main page. Now I'm getting Google results for the B'z Japanese band, because I've been using this for years, and it's hard to untrain my brain. And Firefox doesn't allow the same keyword to be used for search engines and bookmarks, so I can't even add bz as a keyword for the bookmark.
Yes, muscle memory can be a problem, even if alternatives exist.
Note that we got many bug reports from users who complained about the old behavior, because they forgot they had set a keyword for certain words, and reported a bug because they didn't understand why typing "car" was going to "somecardomain.com" instead of searching Google for "car". I think I closed tens of those invalid bugs.
Even if we'd reimplement something like that, we should avoid reintroducing such an hidden and confusing behavior, and I'm not sure how, the only thing coming to my mind now would be to request typing a space after the keyword (basically enter search mode) and confirm an empty search. That's again sub-optimal and undiscoverable. We'll evaluate alternatives.
With all that being said, I also seem to remember that my use case above used to not be supported a very long time ago (only "bz search string" used to work), but became supported later, and it would probably be good to look back why it was added back then, and see why those reasons wouldn't still apply now.
I don't remember a reasoning around that, IIRC it was implemented by mistake. And we didn't think well about the case above where users got really confused when they didn't remember keywords set months or years ago. That bug still exists for bookmark keywords.