There is nothing inherently insecure in browsers. But they do have lots of security problems, mostly they come from trying to support lots of different formats,

There is nothing inherently insecure in browsers. But they do have lots of security problems, mostly they come from trying to support lots of different formats, plugins, programming languages and hence acquiring every security flaw of each and every one of them used (see the vulnerability in pnglib for example) and their sand-boxing is currently very very poor.