If you have also activated the search via the address bar, then after entering a character, a suggestion will be made with the standard search engine. In addition, all installed search engines for changing the search are displayed below. If you would like to deactivate this search engine selection in the address bar , you can do this with a change in about:config.
Disable search engine selection in the address bar
This is what it looks like by default with the entry for it.

Regarding about:config I had already written an explanation here.
about:config explanation
This explanation also serves to provide a better overview of when and what something has already been changed.
So I recommend checking them out.
And this is how the change is made in about:config.
First, start Firefox and then type in the address bar at the top
about:config
and then presses the Enter key and confirms the warning message by clicking on the "I am aware of the dangers!" button.

Then make the following entry in the top left under Search:
browser.urlbar.oneOffSearches
(It is best to copy the name of the entry and paste it back into the bar at the top)
On the right side, under Value, the standard entry is true .

Disable search engine selection in the address bar
Now double-click this value with the left mouse button and change it to false .

Then simply close the about:config page again.
The change is applied immediately by Firefox.
This is what it looks like without the search engine selection:

If you want to change that again, simply set the changed entry to true again by double-clicking.