I’ve been using Firefox snap for quite a while now. Until recently, it was all OK with opening URLs in Firefox right from KeePassXC, which is also snap. A couple of days ago, a brand-new version of KeePassXC has been released, and it is great except for the appeared issue with inability to open URLs. So it looks like this is not a Firefox-snap-related issue, and rather associated with KeePassXC.
And yes, the issue is quite annoying, though it’s not critical.