A "highly critical" Firefox 3.5 vulnerability.

[JillSwift]

Mozilla Firefox Memory Corruption Vulnerability (http://secunia.com/advisories/35798/)

I'm not sure how serious to take the "highly critical" part. They offer a simple work-around, though:
Set "javascript.options.jit.content" to "false" by opening about:config.

[ukripper]

Or use noscript plugin to disable javascript running in the background for the untrusted sites.;)

[Eviltechie]

Some guy said he had to do that on twitter this morning, only that that option didn't exist.

[ghindo]

Mozilla's usually pretty good about patching these things ASAP. We'll see how long it takes them.

[aysiu]

I think "highly critical" refers to the severity of the flaw, not to the likelihood of it being exploited, especially since it relies on social engineering (you have to be tricked into viewing a malicious page).

[Pogeymanz]

Some guy said he had to do that on twitter this morning, only that that option didn't exist.

You can create your own entries in about:config. I recommend you tweet this guy and let him know. (I don't know how twitter works at all...)