http://www.mozilla.org/security/announce/
Interesting to see that their share is falling month by month too. I gave up using Firefox as it had constant issues and updates and above all needs NoScript etc. to keep the browser safe.
I think most software companies announce their own security advices. I thought you meant 3rd parties identifying security weaknesses that Mozilla were not aware of.
I would expect their market share to fall because Chrome is a good competitor.
I don't use NoScript, tried it and didn't like it. Currently I use AdBlock Plus+Element Hiding Helper, BetterPrivacy, BlockSite, Flashblock and Ghostery.
My test for effectiveness of that lot is the infamous DigitalSpy forum....nothing gets through.