Mozilla has been notified of a security exploit and plans to release Firefox 3.5.1 soon. There has also been a reported slow start-up issue on Windows because of the way Firefox accesses temporary files to obtain randomness. Firefox 3.5.2 will be due at the end of July according to Mozilla Links.
I think this just goes to show that Mozilla takes security very seriously and they are making this security update priority one. I’m sure that they are working on the slowness issue as well. Mozilla Links reports that even though a fix might be contained in 3.5.1 it may not solve all slowness issues. That will be addressed in later builds so that 3.5.1 can get out the door.
More to come on this and more from Mozilla.