I don't think that it is possible to pin a custom search engine to a tile. It is possible to add a custom search engine to the drop-down list of the location bar that opens if you type a '@'. You can do that by double-clicking in the keyword column in "Options/Preferences -> Security" and specify a keyword that starts with a '@' (e.g. @youtube or @yt for a YouTube search engine).

cor-el

You can do that by double-clicking in the keyword column in "Options/Preferences -> Security" and specify a keyword that starts with a '@' (e.g. @youtube or @yt for a YouTube search engine).

But you cant add a search engine if it doesn't exist in the add-on page. Not to mention i don't feel good about installing those since i can't see what it does and also generally something discouraged by Mozilla.

I think i have the same problem like OP in that, you can create a custom search by right-clicking in the sites own searchbar, but you cannot edit it as it doesn't show up in the "Options/Preferences -> Security" tab and neither is it showing up as bookmark.

Is this maybe an oversight or a planned thing for later?