Hi Meg! Sorry for the lack of instructions. I realize I might not have done well in that regard. You're right, the change to WebExtensions made it harder to access the search engines that you have in Firefox, so it's not automatic anymore and, therefore, definitely not as intuitive (I wish it was!).

I'll make a note to improve this in the future. I'll also try to explain now. :)

I couldn't understand if the popup was not appearing at all for you now, or if the problem was only that only a few engines appear now. For the second case:

If you go to the add-ons page that has your Firefox extensions (Ctrl+Shift+A is the easy way to go there) and then open Swift Selection Search (SSS) you will be in the "options" page. You should see a lot of options below the add-on description, but the most important ones are at the top under "Search engines": the table of search engines and a button labeled [Select "search.json.mozlz4"] below it. That button lets you give SSS the file that contains the search engines present in Firefox itself. You have a few instructions to the right of the button on where to find this file. After that you should have your engines back, hopefully.

If you have trouble with anything, you can contact me at daniel.rb.lobo+sss@gmail.com and I'll try to help. :)

Regards,
Daniel