The Mozilla Foundation is close to releasing a security update to its popular Firefox Web browser, the group said Thursday.

Firefox 1.0.3, which is being called a "maintenance release" that will include security and stability fixes -- but no new features -- is now in its fifth release candidate (RC) version since Saturday, Mozilla engineer Aza Dotzler announced.

"We're getting ready to wrap up this 1.0.3 release," Dotzler wrote on his blog. "I'm gonna be very cautious about calling any thing a "final" candidate, but this one feels close."

The biggest fix in 1.0.3 will be to eliminate the JavaScript bug disclosed last week that leaves Firefox (and its Mozilla suite sibling) vulnerable to attack.

The Foundation is planning to update Mozilla to 1.7.7 to fix the same problem.

Also this week, another test version of a new popup ad blocker was released. Last Friday, the Foundation posted a beta edition of a blocker that would stymie Flash- and Java-based popup ads, but not interfere with legitimate popups, which many sites use to display information or forms.

The new extension is, said Mozilla's Dotzler, "slightly less draconian" in that it may still let some unwanted popups through, but should block far fewer legit popups.

The extension can be downloaded from here.