I installed Firefox v3.0.10 yesterday, and without running CCleaner, I was unable to log in to any of my sites automatically.
Maybe you ran CCleaner before updating Firefox?
Once again, without running CCleaner, I went back online and all my login details were gone again.
It's possible this bug corrupted your profile in some way which prevented Firefox from saving passwords again.
There are problems all over the internet at the moment with Firefox.
If there are a lot of people reporting losing their saved passwords, ever considered maybe these problems are being caused by CCleaner?
Just out of interest, I have no problems with Opera.
Operas settings are saved to a different folder and it has a different way of storing saved passwords. It's very unlikely the same bug would affect both browsers.