After googling around for some weird error I was seeing in my FireFox console triggered by a script I've been working on, I finally stumbled upon the root of the issue in Bugzilla, in the form of Bug # 236791.
As of this writing, this bug only has 9 votes. It is caused by altering via scripting the focus() of a text input box. Someone pointed out that adding autocomplete="OFF" as an attribute of an <input ... /> element makes the error go away, which I've verified.
If you're a developer and care to make this bug go away in a near future, please vote for bug # 236791 in the Bugzilla system.