You can set some browser-unrelated program or script as your desktop environment’s default browser, for example I wrote a Zsh script that creates a KDE dialog and asks me to copy the URL to the clipboard.

I’m not currently at my PC, but if you want it I can paste it in a comment here when I get to it - it shouldn’t be too hard to translate it to Bash, either.

Other than that? /usr/bin/true is a pretty nice default browser for applications to start without your consent, very minimal and lightweight.

Other apps than what?

You can start Firefox without the --no-remote flag. And it would allow external processes to open links.