After much frustration trying unsuccessfully the normal ways of setting a browser (brave-browser) as the default in Lubuntu, I found that printfgabriel posted the work-around:
Configuring x-www-browser in Alternatives, XDG’s default-web-browser and Lubuntu’s default internet browser application in Session Settings is not enough. Turns out Chrome looks at /etc/xdg/xdg-Lubuntu/mimeapps.list and you have to edit it as sudo and replace the instances of the “firefox” string with your browser of choice.
I replaced all instances of "firefox" with "brave-browser" and that worked. Whew!
