How safe is the android Lock Screen with password?
I wanted to know, so I tried entering my password incorrectly repeatedly. (Motorola Droid 3 - Android 2.3.4)

After 5, 10, and 15 attempts I was given a 30 second "cool down" where I could not try again. 

On the 15th attempt I was informed that if I entered my password incorrectly 5 more times I would be forced to re-sign in with my Google Account.

So after 20 failed attempts, you can no longer try to enter a password - you must sign in with your Google Credentials.

I would say you are reasonably secure from a brute-force password log in at the password screen.

This does not take into account any other methods - just attempting to brute force.

This answer may address methods that could be used to bypass the lock screen.

To specifically address your concern about Firefox for Android's stored passwords - you can disable Firefox Sync on a lost tablet (Mozilla's Instructions) by changing your sync password. Mozilla also suggests changing all of your passwords after it.