I use vimium in Firefox and so my default key bindings are the plug-in ones. I push 't' to create a new tab, for instance. If I want to use the website key bindings I have to to into "insert mode" ('i'), or I opt into specific keys by site.

I do like when websites use ctrl-k -- it means nothing to my plug-in so websites always get it, plus it helps with key binding discovery.