This is an offshoot discussion and I have nothing against DDG or bang commands -- just presenting another side of convenient searches.
> I know Firefox had install-your-own search plugins forever, but the total lack of friction of typing “!yt badgerbadgerbadger” to get specifically YouTube results was fantastic.
I find "use your own search" to be much less friction than going through DDG since it takes me right to results from the custom search, rather than having to redirect through DDG before going to the results. (Edited to add the following) I'm not going to lie and say that it doesn't make me a little paranoid to think of EVERY search I do going through one provider regardless of their current privacy stance.
I've used them since 2002ish in Opera, continued using them in Chrome, and now I use them in Firefox.
AFAIK you don't even need "install-your-own search plugins" -- just right-click on the search box on the site you want to search, and add keyword.
For example, I have "aj" for Amazon Japan and "au" for Amazon US, so I can just hit cmd-t for a new tab and type "au pretzels" directly in the address bar.
This has the advantage of letting you define your own, too.