I've been trying to figure out why I was having problems with some yahoo functions, and have tracked it to LinkThing 2.0.26. This is in Safari 5.1.4, I haven't tried it in another browser.
When in the Mail window, I click Options>Mail Options. In the left sidebar are several links, such as General, Signature, Filters, POP and Forwarding.
When LinkThing is enabled, every time I click one of this links, I get an error page. After disabling LinkThing, the links work correctly.
I expected the problem to be from an extension like AdBlock or Plugin Customs, not LinkThing, but it's definitely the latter.
The first image shows the sidebar and what should be seen after clicking POP and Forwarding. Second image shows what is seen with LinkThing enabled.
Third image shows my LinkThing settings (default settings used in the other tab).