The safest password is the one you don't remember. That said, any saved password is vulnerable to compromise, no matter if it is saved in a Firefox addon or not.
I used WebMail Notifier for a long time, and I loved it. Now I use Thunderbird
to check my email and I let it save my passwords, too.
However, the only realistic way somebody could potentially get at your saved passwords (or mine, for that matter) is to be physically sitting at your machine. In Thunderbird, you can go to "Saved Passwords" and click "Show passwords" to see the passwords you have saved. But that's not a big deal to me since I'm the only one who has access to my machine.
The "hacking of webmail" doesn't have much to do with your saved passwords, but more to do with companies not protecting their data well and people re-using passwords. See my posts in this thread for more info: http://www.bleepingcomputer.com/forums/topic409063.html
Here's a list of things you should
be wary of, including but not limited to: using open Wi-Fi access points (unencrypted), using the same password for multiple accounts, out-of-date software (including anti-virus), downloading torrents, porn, etc.
In summary, it is perfectly fine, in my opinion, to use a webmail notifier (assuming you trust the author of the software, that is) or a dedicated email client like Thunderbird. They have the same level of security.