When I logged in, firefox started with this link: https://cgit.kde.org/?p=kate.git&a=tree&f=addons%2Fkate%2Fhelloworld I've never used kate and only started it once out of curiosity a few months ago. So, I never installed any plugin either.
Hi, I really wonder how you got that link opened since a search in the source code for "kate.git" only showed a match in README.md and AUTHORS. I also searched for helloworld and got zero matches (the plugin has been removed). There has to be some distro specific thing or something left behind from previous browsing...
I don't use that browser profile at all(in the home directory: .mozilla). I always use sandboxed(firejail) firefox instances which look very different and are in another subdirectory.
Linked this on the manjaro forum: https://forum.manjaro.org/t/kate-or-one-of-its-plugins-opens-a-helloworld-test-in-firefox-at-desktop-startup/38371
I am not aware of any way we can cause that ;=) We don't install any "autostart" like stuff nor some stuff that opens web links. If that happens for you with a fresh install, please report it to your distribution.