Actually, at the time, I would've been perfectly happy to have just imported the passwords into Firefox!
But I don't think it is able to import them, at least not on my machine. I'm using the latest Chromium/Firefox on the latest Ubuntu, and I just had another look. When I select the option to import data from another browser, I get a dialog that says:
Import Preferences, Bookmarks, History, Passwords and other data from:
Chromium
When I select Chromium, I see a list of things I can import:
Select which items to import:
[x] Cookies
[x] Browsing History
For some reason, "Passwords" does not appear in the list, and when I browse to a site in Firefox, it doesn't use the password that Chromium had stored.
Maybe this is an OS-dependent thing?