In Ubuntu the version of Firefox that gets downloaded through apt-get is 18.104.22.168. But Firefox is up to version 9.01 already.
Why does apt-get only get version 3.6? Why is Firefox already up to version 9.01? Is 3.6 not secure anymore?
Canonical intends to provide support for Ubuntu 10.04 until April 2013 for the desktop version, and until April 2015 for the server version.
You should be fine. Your release is still covered by the security team. Upgrading to 11.10 would not hurt, though, if you want to use more recent features.
UPDATE: As stated in the comments, you are using 10.10. "Maverick Meerkat 10.10 will be supported until April 2012." So it's time for an upgrade soon :-)
If I go to Ubuntu Software Center and look at the Firefox version I got from there, it says I have version 9.0.1 - so apparently at least Ubuntu 11.10 has the latest version in its software repositories. What Ubuntu version are you running?
If you use up to date version of firefox from supported version of Ubuntu, then you don't need to worry.
The "clean 3.6" version of Firefox might not be secure, but Ubuntu provides security fixes. Look at the version of the firefox package. My is for example:
You can also check the changelog of your package, to see when and what has been fixed:
If you use Firefox not from the repository (i.e. tar.gz from mozilla.org) or if you do not install security updates, then you are on your own and there is a potential risk. But why would you do so, right?