I also have this error for the latest KDE Neon install. I can reproduce this error both with the firefox from the ppa (107) and the one from flatpak (106.0.5)

I have tried to link the /usr/lib/mozilla/native-messaging-hosts/org.kde.plasma.browser_integration.json to ~/.mozilla/native-messaging-hosts but it still failed.

(In reply to Daniel Bruckner from comment #0)
> Trying to connect the Plasma-Integration-Addon doesn't work. Error: "Failed
> to connect to the native host"
> 
> KDE-Neon fresh install
> SOFTWARE/OS VERSIONS
> Firefox: 106.0.5 (Installed from ppa:
> org.kde.neon.net.launchpad.ppa.mozillateam) 
> Linux/KDE Plasma: 5.26
> (available in About System)
> KDE Plasma Version: 5.26.2
> KDE Frameworks Version: 5.99.0
> Qt Version: 5.15.7
> 
> ADDITIONAL INFORMATION

(In reply to aceserraon from comment #1)
> I also have this error for the latest KDE Neon install. I can reproduce this
> error both with the firefox from the ppa (107) and the one from flatpak
> (106.0.5)
> 
> I have tried to link the
> /usr/lib/mozilla/native-messaging-hosts/org.kde.plasma.browser_integration.
> json to ~/.mozilla/native-messaging-hosts but it still failed.

I have also checked the file permissions for /usr/bin/plasma-browser-integration-host and found that it is the following (from ls-l)
-rwxr-xr-x 1 root root 244016 Oct 26 23:12 /usr/bin/plasma-browser-integration-host

Is this intentional?

After updating Firefox on ver. 107 everything works well.

Still failing for me with Firefox 108.0+build1-0ubuntu0.22.04.1~mt1 in Neon, even reinstalling the plugin

Operating System: KDE neon 5.26
KDE Plasma Version: 5.26.4
KDE Frameworks Version: 5.100.0
Qt Version: 5.15.7
Kernel Version: 5.15.0-56-generic (64-bit)
Graphics Platform: X11

Having the same issue. Confirmed the application is installed at /usr/bin/plasma-browser-integration-host, and the native application manifest exists at /usr/lib/mozilla/native-messaging-hosts/org.kde.plasma.browser_integration.json.

This is with the native apt package installed. I believe it's an issue with the plugin/app itself, since when launching with a flatpak (which I expect to not be able to connect to the native host), and it has an extra line, "No such native application org.kde.plasma_browser_integration", which doesn't appear in the apt version.

Operating System: KDE neon 5.26
KDE Plasma Version: 5.26.4
KDE Frameworks Version: 5.100.0
Qt Version: 5.15.7
Kernel Version: 5.15.0-56-generic (64-bit)
Graphics Platform: X11
Processors: 2 × AMD A6-9220C RADEON R5, 5 COMPUTE CORES 2C+3G
Memory: 3.8 GiB of RAM
Graphics Processor: AMD STONEY
Manufacturer: Google
Product Name: Grunt
System Version: rev6

"No such native application org.kde.plasma_browser_integration" suggests the config for the messaging host isn't in the right spot. Wonder where that would go for a snap.
That said I won't support this since Canonical has known for five years what a shit idea this is and they only now desperately try tk come up with a solution. Just use the apt package like a sane distro.

(In reply to Kai Uwe Broulik from comment #6)
> "No such native application org.kde.plasma_browser_integration" suggests the
> config for the messaging host isn't in the right spot. Wonder where that
> would go for a snap.
> That said I won't support this since Canonical has known for five years what
> a shit idea this is and they only now desperately try tk come up with a
> solution. Just use the apt package like a sane distro.

I don't get where do you think the problem is. Should I report the problem to neon, ubuntu, firefox ppa or what?

I think this bug is a duplicated of BUG 468862 , a problem with apparmor

*** This bug has been marked as a duplicate of bug 468862 ***

Can be solved with

> sudo ln -s /etc/apparmor.d/usr.bin.firefox /etc/apparmor.d/disable/