The only way to hijack key presses that I can think of are javascript. Discourse based forums are the worst about this. Like this. Hit ctrl F and see what happens.
I started using the Disable JavaScript addon because I was so sick of this shit. With that addon there is a toggle so you can flip javascript on and off per site. It remembers your choices. It does wonders for news sites. No ads, loads a ton faster.. Javascript is eval.
That was just how I got started. Now I use NoScript that just does a blanket ban of all javascript and you have to enable certain scripts per site. With the first thing I linked to it is javascript on or off. With noscript you can allow certain things per site. This breaks nearly everything. But after about a month of training I barely have to touch it and I love it.
Look at all this worthless shit Noscript blocked on CNN.