Use a different browser. Firefox developers have decided the current behavior (match domain + username) is the only way (which was different in Firefox 2.x). See the Firefox bugs:
Bug 263387 passwords should be stored for host+path and not just host
Bug 444333 Password not saved for multiple login within same domain
Bug 436275 wrong password filled in when same login is used for different forms
All those bug reports are marked as "wontfix", meaning the developers think is not really a problem or is too hard to solve or it will create more problems.
Finally, I don't know of any Firefox addon that allow to store the same username for the same domain for different sites/applications (at least not those addons which use the Firefox Password Manager, maybe those replacing or using an external password storage can do it).