I'm running the latest updated neon user edition with plasma 5.18.2 and whenever I start Firefox it prompts to set as default browser.

Even after clicking YES or setting default in Firefox preferences it still prompts next time.

I have filed a bug here https://bugs.kde.org/show_bug.cgi?id=418311 hopefully it gets noticed.

Anyone else facing this issue or is it just me?

lol
This problem for about a year.
On Arch Linux with Plasma, Chromium every time asks about default.

xenobeat wrote:lol
This problem for about a year.
On Arch Linux with Plasma, Chromium every time asks about default.

So this does indicate the problem lies in KDE/Plasma and not the browser itself.

well, I still wouldn't be so hasty to post blame. Have you tried in other environments?

Also, check what you have set up in System Settings > Applications > Default Applications > Web Browser.

A little googling turns up https://askubuntu.com/questions/79305/h ... lt-browser which has additional suggestions for ways to set the default browser. You may be able to modify some of those commands to examine what the setting is which may be helpful in diagnosing why it isn't updating.