In previous versions of Firefox, if I wanted to use the default search engine to search a particular word present in the current page, I could just select the word and press the left mouse button and then drag it a bit before releasing the left mouse button, then the search results would pop up in a new tab.
It seems that the above feature has been removed after the recent update, hopefully it will be added back to the browser or as a feature that can be switched on and off manually.