Firefox should not be changing the default search engine without the user selection. There are some circumstances where third party software will change the default, but I suspect that may not be the case here.
If this happens again, please file an i...
You can disable search engines in the Settings, under Search -> Manage alternative search engines.
If you disable the search engines there, they will not be used nor be available in the browser.
They'll still be listed within settings because a user ...
If you have text in the address bar, you can triple-click it to select all (or use other means to select all) and then drag the text to the bookmarks toolbar.
I'm not quite sure what you mean by the new tab interface, but I think you want to go into settings, and under the Search section, look for "Search Suggestions" and then toggle "Show search suggestions ahead of browsing history in address bar results...
It doesn't completely solve the issue, but you can right-click and select "show in folder".
This is a duplicate of https://connect.mozilla.org/t5/ideas/searching-for-a-bookmark-show-the-folder-where-the-existing/idi-p/8655
