There was one of those exploits that you hear about found on Firefox 16.0 the other day. As a result, the recommendation is that if you have Firefox 16.0 to downgrade to version 15.0.1 and wait for a fix.
You can check your version by doing the following:
Click About Firefox
If you have version 15.0.1 you're safe.
If you have version 16.0, you will want to downgrade to 15.0.1
The good news is that they're working on a fix and supposedly will have one out by today, October 12th, 2012... but you never know.
For more information, you can read up on the problem here, or look into their Security Blog here.
What the bug does is it allows an evil doer to see what you have done since you started the browser. So if you plan on waiting it out, don't hit the banks.
That's what happens when you rush software out the door.