http://secunia.com/advisories/12983/
They have in the past. But you have to understand, security research is typically done on the more popular things. Thats why firefox and IE are always under the spot light.
Just because you don't run into many well known vulnerabilities, doesnt mean one won't become an issue.
What exactly are you looking for in a browser? You can speed up firefox by unloading all of the unnecessary extensions.
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I've never used webkit directly, but it is apparently what IE and FF are both built from. So it might be worth checking out.