you want is this feature built natively into Firefox?
Yes
but considering how well extensions emulate it
A single search engine management system is more convenient than two parallel ones. Firefox already has the ability to add and manage search engines for the address bar, just need to make them available from the context menu.
Currently, there is a context menu option when text is selected that allows searching with the default search engine. It would be better if users could replace this option with a `Search "word" on...` submenu that lists installed search engines.
The option to contextually search for images as well seems amazing, but wouldn't it need some kind of information on whether the search engine supports it if at all?
I would appreciate it too, but considering how well extensions emulate it, it probably isn't a big priority for Mozilla.
Sadly, it's still inconvenient to have an extension inserting a new menu at the end instead of replacing the default one (having to edit a userChrome.css file doesn't help too).
At first I thought, nah, it's enough for the browser to offer your default search engine in the context menu and, if you want expanded functionality, go ahead and get or create an extension. After a bit more thought though, I believe this would not be too difficult to implement nor for users to configure/manage. Kudos awarded.
Just to expand a bit here, my use case is searching for terms on different dictionaries (urban dictionary, jisho, wikipedia, etc.), so it's somewhat inconvenient that I have to open a new tab or install an extension when I already have these dictionaries installed as search providers. And even with an extension, the menus become duplicated (default provider + extension provided menu item), since the default one is still there occupying space, while a submenu would avoid it entirely.