I was going to say there is a link about how to add Startpage below the search box here https://startpage.com/ but it sounds like you found that method.

Now that it's installed, you should be able to customize it in the searchplugins folder. All the action is usually at the end of the file where you see the fields defined.

I think the built-in plugins use HTTPS, but more generally:

You cannot directly customize built-in plugins. You can copy their code as a starting point for your file. You can access the bundled .xml files by pasting this in the address bar and pressing Enter:

chrome://browser/locale/searchplugins/

In your new .xml file, please make sure to change the ShortName (max 16 characters). Otherwise, the built-in search plugin with the same ShortName takes precedence over your version.

When you copy a completely new file into the searchplugins folder by hand, I don't know how soon Firefox will detect it. You may need to exit and restart Firefox to get it to re-read the folder and list it on the Options/Search page.

Does that work?

(I'm not familiar with the PowerShell method. My eyes glazed part of the way through the script.)

Also, see this MDN article on OpenSearch plugins, which are more standard than MozSearch plugins: https://developer.mozilla.org/Add-ons/Creating_OpenSearch_plugins_for_Firefox

I used the button in Startpage's settings dialog. Didn't saw it at first.

It seems that the folder searchplugins and the files search.json and search-metadata.json are ignored in the current Firefox version. Deleting these doesn't affect installed search engines (I restarted FF after each change) and they are not created with a new FF profile, even after installing more searches. Customized search engines are saved in search.json.mozlz4 but this file looks pretty weird...

Anyway, thanks for your help so far and the tip to change the ShortName!

The search.json.mozlz4 file has replaced the search.json, search-metadata.json, and search.sqlite files used in older Firefox versions. This file is compressed in the same way as the bookmark backups in the bookmarks folder with LZ4 compression.

Just logged in to say THANK YOU MOZILLA! Thank you for making another part of my browser UNCUSTOMIZABLE. I just wanted to edit the XML file of a search plugin i just downloaded from the (poorly maintained) mycroft.mozdev.org repository. And i ended up in being annoyed in the support forum. What's the point in introducing a file extension .mozlz4, please? Couldn't you choose something that at least could be opened up by 7z? You desperately want power users to move on, right?

Those changes did concern me, too, as I have a number of search engines that I have customized over the years. But Firefox users can still create their own custom XML search engine at the Mycroft Project and then install it into Firefox, where it will be imported into the new format. The most important thing (IMO) is to have a unique "ShortName" or Firefox may ignore that search engine. Not sure if that applies to all search engines, or just the default search engine that are embedded into Firefox; which are "protected" and can't be overridden by an Add-on search engine. That threw me for a lopop back in Firefox 38 when the file types started getting changed from had been used for years. http://mycroftproject.com/