Thanks. I've found the problem based on the links you provided.

The clipboard API requires a secure HTTPS website to function. The first link you provided uses HTTPS on the main page, but not on the results page. I assume it's the same issue you're having on your local server ( no https ).

I've added some fallback code that seems to work, but the function is long depreciated, and may stop working at some point. Fixes will be in the next release.

👍 sorry I didn't see your edit sooner. I have a fix ready and will have a new release soon

I think this bug may have finally been tracked down, with the help of a user @ the github support site. If you're game, backup your current CS config and install the signed pre-release from here: https://github.com/ssborbis/ContextSearch-web-ext/releases/tag/v1.32a1

It should work fine. Make sure you're on the latest version. v1.31 was a bit of a mess

(edit3 - 10-19-2020) Were you searching a folder or a single engine when you observed the problem last? I've uploaded v1.27.5 which fixed the same problem v1.27.4 did, but when searching a folder as well.

(edit 2) Thanks for the reply. I thought the bug fixed in 1.27.4 would take care of your issue, but there must be another bug. I've tried all the engines you mentioned, but I'm getting good search results using the context menu now. Could you open ContextSearch options -> Search Engines and tell me if you see an orange (FF) next to those engines you're having issues with? If not, could you edit one of them and paste the template / method / etc info? It could help narrow down the problem.

(edit) Nevermind, version 1.27.4 going up. Context menu was not working with Firefox engines

Would you mind opening a ticket at the support site or editing your reply here? Some users seem to be experiencing problems with the latest builds and I could use some input on what exactly is going wrong. I tried adding Yandex to my install and had no problems, and troubleshooting is difficult when I can't replicate the bug.