Some people wouldn't consider these strictly *security* features, but
if you're using Firefox it helps to be aware of
firefox -ProfileManager
and use that to keep stuff separate. Additionally, you can also use
-no-remote and -P <profilename> to concurrently run two or more
firefox instances with different profiles. Cf.:
http://kb.mozillazine.org/Command_line_arguments
Also be aware that even after you've deleted all traditional cookies,
so-called "Flash cookies" (LSOs) may still persist, and sneaky sites
do use those to track you as well.
One add-on that you can use to kill those is this;
http://netticat.ath.cx/BetterPrivacy/BetterPrivacy.htm
(And even after deleting all cookies and LSOs, sites can still tell
what other places on the web you've been to, due to CSS leaking that
info, which may be unfixable, cf. e.g.
http://www.amirharel.com/2009/09/20/css-privacy/ )
Finally, if you use Adblock Plus, you owe it to yourself to also use
Element Hiding Helper.
This will not necessarily make Firefox "more secure than others", and
there are lots of things about Firefox that suck ass, but the above
will, "tighten [its] security", at least for some value of security.
--regards,
ropers