Hi, I just found this thread as I have the exact same problem: Firefox on Linux, uMatrix Extension enabled. But as mentioned above, this extension is unmaintained (which I did not know until I read it here). So I will try to find a replacement and see if this solves (or workarounds) this for me.
I have placed a bookmark to kagi.com in my bookmark-toolbar so I can just click it when a search in the location bar redirects me to the login page. But the search string is lost this way, but I do not need to login again. So I can confirm its not an issue on the kagi.com side.