First, if a shared location does not work, you can put the search engine plugins in your personal profile folder, in a searchplugins subdirectory. (Profiles - Where Firefox stores your bookmarks, passwords and other user data)

Second, you should modify the file name and the search engine name from any built-in search engine to avoid the built-in code overriding your plugin.

These old threads might be relevant:

• Where has the 'searchplugins' folder gone?
• Searchplugins in profile folder no longer recognized

Firefox no longer uses a /searchplugins/ folder in /browser/ where the Firefox program files are installed. The 'protected' searchplugins that come with Firefox were moved to the omni.ja file in Firefox 40 and later. And I wouldn't advise adding anything to the omni.ja folder, as that folder gets updated / replaced with almost every update.

Just a guess or a 'hunch' ... if you are building from source ... Try creating a /searchplugins/ folder in /browser/ and place your searchplugins in that folder.

Another thing to be aware of is that with Firefox 44.0 there was an 'enhancement' made which complicates matters even further. In the Profile folder there is now a search.json file [which appears to list ALL search engines] and a search-metadata.json file [which is for only the default searchplugins] - I have no idea what those files are used for and how they affect "user installed" search plugins which would be in the /searchplugins/ folder in the Profile folder; but I do know that Firefox will automatically re-create search.json file if the user deletes it. I did delete that file when a few of my search plugins suddenly disappeared, when they were still present in the /searchplugins/ folder but didn't appear in the Options > Search --> One-click search engines tab.