I am facing the same issue as fselcukcan. I can use firefox normally, but when another application tries to open firefox while it is already running (for example clicking a link in an email), I get the same error message. When Firefox is closed and an application tries to open it, there are no issues at all. I have tried this on a fully fresh install of manjaro-gnome. Help is apprechiated.
Actually, I want to get into more detail on the issue, because it is more nuanced:
When firefox is open and another application tries to open a website (for example clicking on a link in skype) then it works. BUT
It only works with the first application that tries to do this.
So, when I open firefox and then click on a link in skype and then on a link in discord, the first link opens, the second doesn’t. When I restart firfox and then click a link in discord, it opens, but when I then click a link in skype, it produces the error message.
Opening multiple links with the same application is possible.
Basically whichever application first addresses firefox will be the only application able to address firefox until a restart.