I like new Firefox but if Firefox is removing search bar than address bar should take over all functionality from search bar and it doesn't.In search bar there is easy way to add new search engine with + sign and in address bar there isn't.
Where do you see the search bar being removed? I can't see any announcements, I don't think it's going away. Also, I don't think the combined bar is missing functionalities, at least not the one you're describing.
On Firefox Developer Edition 58, in the menu you can choose between :
Use the address bar for search and navigation
or
Add search bar in toolbar
This decides between combining the address bar and search bar into one, or having a separate address bar and search bar. You can also pick a keyword for each search engine.
Even if you decide to use the combined bar, I just tried adding "+ddg" keyword[1] on Duck Duck Go and type "+ddg test" and it searched for "test" on Duck Duck Go instead of my default search engine.
[1] https://support.mozilla.org/en-US/kb/add-or-remove-search-en...