My idea is to enable adding custom search shortcuts, that are different from the search engines you can add with 'Find more search engines' in the settings. For example, I have added search shortcuts for YouTube and Twitter, like in the screenshot.
This enables me to search quickly in those sites, however, this is not applicable for websites that were not build with modern search functions and we need to use a different method, that is a hidden feature in Firefox. For example in order to search in Twitch, I need to bookmark the search page, replace the search term with %s and add a keyword, like in the screenshot.
The fact that I couldn't find this feature before and the lack of grouping tabs, I switched to a different browser for work, in order to manage my many tickets.
It would be great to have both those functionalities in one place and merged, then more people will find this useful and use it.
They've already added a very simple method for adding custom search engines to Firefox Android, it would be great if they could add it to the desktop version as well.
It already existed before and was visible by default, they disabled probably to favor Google, Bing, and other sponsored search engines; you can make it visible in about:config (add the following entry) browser.urlbar.update2.engineAliasRefresh and set it to true, now you'll see an "add" button next to the "remove" in Settings>Search (your first picture). Let's hope they don't remove the feature completely in the future.
I've been using Firefox since Netscape days, and is very hard to like it nowadays 😞
It already exists they hid it probably to keep sponsored ones happy, to re-enable:
in about:config add the entry
browser.urlbar.update2.engineAliasRefresh and set it to true; now you'll have back the "add" button in Settings - Search, next to the "remove" button in your first screenshot
I've been using Firefox since it was called Netscape, is very hard to like Firefox nowadays, Mozilla is making a great job removing choice from users.