Yesterday, it seems that Google did some work on their API, and suddenly the maps broke for a lot of people.
The exact pathology of the problem is still a bit fuzzy, but it seems to affect mostly FireFox, and the cure is pretty simple:
Delete all "google.com" cookies in your browser.
This is a pretty unsatisfactory solution for your users, but, until Google addresses the issue, this is all we can do. Thankfully, the affected users may comprise a small and technically literate group.