An update to fix stability and security issues has just been released. Install it by opening Firefox (some insist on calling it Mozilla or Mozilla Firefox - isn't that like calling someone by their whole name all the time?). Go to Help - Check for updates and follow the prompts.
You may then be prompted to update Adobe Flash - again, follow the prompts and let it happen. Be sure to uncheck any other "free" stuff they want to install along the way, like the McAfee Security Scan.
I've not been a huge fan of Firefox until version 3 - now I use it more than ever. It's especially useful when Internet Explorer 7 or 8 refuse to cooperate with certain sites. The first thing I tell people who ask why they can't do something online or even get online is to try doing the same thing in Firefox. Usually, Firefox just works. I install it on every computer I touch, so if I've seen yours you have some version of it already.
For those who want to download a fresh copy of Firefox, it can be found here: http://www.mozilla.com/en-US/
And for those who want to know the changes from the previous version, read about it here: http://www.mozilla.com/en-US/firefox/3.5.3/releasenotes/
This is not an optional update, considering the security and stability fixes - do it now.