The searchplugins folder may still be used at least temporarily. It may be almost as easy to add the plugins by visiting the sites.

The new files involved are

• search-metadata.json That is some settings including the display order
• search.json.mozlz4 That is not human readable. It is compressed in a none standard way. It incorporates the searches engines initially &/or previously stored in the folder searchplugins

Edit I posted that from a phone. Now correcting spelling typos

The searchplugins folder is only used in the Firefox profile folder and anot not in the Firefox installation folder because the built-in search engines are in one of the omni.ja archives.

You can inspect the built-in default search engines by pasting this chrome URI in the location bar.

chrome://browser/locale/searchplugins/

cor-el said

The searchplugins folder is only used in the Firefox profile folder and not in the Firefox installation folder because the built-in search engines are in one of the omni.ja archives. ...

Try copying the search.json.mozlz4 file to transfer add-on search 'plugins' between Profiles.

As of Firefox 45 it appears that a /searchplugins/ folder isn't created when an add-on search engine is installed in a new Profile; either a fresh installation or a newly created Profile.

I played with a new Profile a bit, and it looks like the search.json.mozlz4 was changed when I added a 2nd Mycroft search plugin; but only 1 kb was added to the file size. And I can't find the IMDb.xml search Plugin file which was installed. Plus, I'm not seeing a time-stamp for any other file changes in the Profile folder at the same time as the search Plugin installation, as when the mozlz4 was changed.

Typically the search.xml files are 2kb to 3kb in file size, so a bit of compression with mozlz4 json is within reason - but I can't read enough of that file in a text editor program to "see" much. I wonder if there is a mozlz4 'de-compression' and maybe 're-compression' web page or utility program to allow the user to 'read and edit' that file type?

Overall, the lack of the /searchplugins/ folder and text-based xml files prevents the user from editing their added search engines; or so it seems right now.

I found the Bug report for that change in Fx45. Target Milestone: Firefox 45 https://bugzilla.mozilla.org/show_bug.cgi?id=1203167

And another under-estimation of the end "user side of things". https://bugzilla.mozilla.org/show_bug.cgi?id=1203167#c47

I have 3 Google search plugins that I have edited over the years, and needed to change the "short name" in Firefox 38 to allow them to be "recognized" and appear in the Search Bar. The "default" plugins take precedence over any add-on search plugins of the same "short name". I was able to identify each of those 3 by having a unique icon.

But it looks like there is a solution to installing "user edited" search.xml files posted here: https://bugzilla.mozilla.org/show_bug.cgi?id=1236498#c7

I wonder if Alice0775 White will come up with an add-on to make 'editing' a bit more user-friendly.