Up to Firefox v60, when keeping +<ARROW UP/DOWN> pressed, search engine selection stopped at the beginning and end of the search engine list.
In newer versions of Firefox the list is wrapping.
Is there a configuration setting to stop this list from wrapping?
I’m afraid there’s no option to go back to the old behavior. Since there’s no visual indication of where you are in the ordered list of search engines, this was considered to be an improvement over the older behavior, since you’re able to continue to your desired choice in a continuous cycle if you happened to hit past it.
What a shame.
I think the old behaviour was advantageous because there were two anchor points to easily navigate to.
You could set your preferred two search engines to each end of the list. Then you could easily use them by keeping the CTRL + ARROW buttons held down for a second and start entering your search term.
I believe the new behaviour to be a disadvantage. I find myself spending more time finding the right search engine than being focussed on my search term and the information I’m actually dealing with.
Sounds like a bit of muscle memory is fighting you here 
The new situation is effectively a net improvement in your case, because now you can just hit Ctrl+Arrow Up once to select the engine at the end of your list and Ctrl+Arrow Down once to move back to the first engine in your list, without needing to select all of them by keeping your arrow key depressed.
That’s true. Muscle memory is the key here.
In fact, your suggestion won’t work when the search bar was left with one of the other search engines selected the last time it was used. While the other approach always worked.
In fact, that’s what the [HOME] and [END] keys on the keyboard had been invented for originally for typing.
For example: Second search engine would be:
Second last search engine would be: