Flatpak app help menu and about... links don't do anything

I get the same behaviour across all the flatpak apps I've tried so far (actually, also in the Discovery app). When I use the app's Help menu to try and open the app user guide, nothing happens. Same if I click on a hyperlink in the app About screen - or any of the hyperlinks shown in the Discovery app.

Is there a permission I need to grant in flatseal or something before app help and links start working?
Solution
The solution was to change the default web browser in Settings > Default Applications. I think this behaviour came about because I disabled Firefox in favour of using Chrome (I know, but Firefox's profile management is so bad and my workflow requires separate profiles for different use cases), but the default web browser was still set to Firefox even though it's no longer working on my system.
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