Firefox 61: add search engine in Page Actions menu
Mozilla removed the classic search bar from new Firefox installations with the release of Firefox 57 but did not touch the search bar in existing Firefox installations.
Mozilla added so-called one-off searches to the address bar which users of the browser can make use of to run searches on different search engines directly from the address bar.
The search bar is still included in the customization menu, but it seems likely that most new Firefox users won't add it to the browser. Likely, because they would have to discover the option first to do so and would have to find the built-in address bar search lacking.
The removal of the search bar removed the option to add open search providers to Firefox. The search bar highlights open search providers when you visit pages in the Firefox browser and provides you with an option to add the search engine to the browser.
With the search bar no longer displayed, users were not informed anymore about open search providers found on a web page. While it is possible to add new search engines in the Firefox preferences, doing so is not nearly as comfortable as clicking the mouse twice to add a new search engine in the frontend of the browser.
Firefox 61: Page Action menu add search engine
Mozilla is aware of the issue and plans to introduce a new option in the Page Actions menu to resolve it. The Page Actions menu was introduced recently in the browser. You activate it when you click or tap on the three dots in the Firefox address bar.
Starting in Firefox 61, a new Add "search engine" to One-Click Search action is available that Firefox users may use to add an open search engine to the browser.
The current version of the feature is already live in Firefox Nightly. Note that the Page Actions menu won't highlight that a new Open Search provider was found on the active page; a core difference to the search bar which highlights found search providers.
Now You: how do you search in your browser?