<-- Background -->
I have an alternate, less-intrusive idea for you here (at least if you're running an ad blocker, as so many of us are for our own sanity/safety). I like using existing extensions/scripts as much as possible to avoid the bloat of a whole extension for just one feature (worst-case scenario) so this solution works under this principle.
Adblock, and its variants/successors (uBlock is my weapon of choice), have the ability to block web page elements, including
<link> elements, which is used for autodiscovery of OpenSearch Descriptions (OSDs), the XML files that contain the information which permits auto-adding search engines and causes us these headaches. I say "permits" because it's hardly mandatory, as, so far as my research has shown, Firefox simply reads this information and makes it available for easy addition under the Search Engines dropdown box, rather than quietly auto-adding it like Chrome does.
The use of the feature is described in the Opensearch specifications in multiple places:
(ignore the specific subtltle of this section for our purposes as it's just an example of it in use)
<-- The Solution -->
As it states that OpenSearch Descriptions (OSDs) have a unique type, we can filter them out with the following AdblockPlus/uBlock rule:
I've tested this and the rule shows the correct match on my test sites (filehippo.com etc) and the search engines are no longer auto-adding, so I believe this is a full solution.
A quick note on the history I've found behind this: Chromium's engineers have labeled this "WontFix" several times over the years (a power-user disable option/flag was requested multiple times) stating that this is considered a niche issue since the feature is "generally useful", their stance is that niche issues should be solved by extensions or 3rd-party scripts rather than by the devs adding countless flags and the like to cater to all whims, so basically what we're doing here is just in line with their preference and keeps it nice and manageable.
Best of luck! If anyone else tries this let us/me know how it works!