This is another good reason to use the Opera, and one of the key reason that Opera users and devs have been arguing for a very long time against plugins and extensions.
There can be little doubt that Opera is the safest browser out there, particularly if you like to routinely browse questionable websites; while the safe sandbox of userJS, userCSS and widgets in addition to the plethora of out-of-the-box features means that there's very little need for extensions anyway.
Firefox is a great browser, and much more secure than IE, but since its growth in popularity combines with the number of malicious extensions out there it can no longer be considered to be a completely secure browsing environment.
Don't take my word for it, check out Secunia's own advisories.
sebt
The first rule of intelligent tinkering is to save all the parts. -- Paul Erlich