Bug 1388838

Thanks for the report. What desktop environment are you using?
Please attach output of following commands:
xdg-mime query default x-scheme-handler/http; xdg-mime query default x-scheme-handler/https

Desktop environment
plasma-desktop-5.7.5-1.fc24.x86_64


$ xdg-mime query default x-scheme-handler/http; xdg-mime query default x-scheme-handler/https
mozilla-thunderbird.desktop
mozilla-thunderbird.desktop

Could you please check the default Web application in KDE settings?
Should be somewhere there: https://userbase.kde.org/System_Settings/Default_Applications

You might noticed that the output of xdg-mime showed that your default handler for http and https is the Thunderbird. This is strange, because the mozilla-thunderbird.desktop file we provide handles only message/rfc822 and x-scheme-handler/mailto mimetypes. 

You could also check content of mozilla-thunderbird.desktop (usually in /usr/share/applications/mozilla-thunderbird.desktop, but can be also in your home directory eventually, try to lookup the file in home too) for the MimeType value.

KDE Plasma settings are fine

cat /home/user/Desktop/mozilla-thunderbird.desktop

[Desktop Entry]
Version=1.0
Name=Thunderbird
GenericName=Email
Comment=Send and Receive Email
Exec=thunderbird %u
TryExec=thunderbird
Icon=thunderbird
Terminal=false
Type=Application
MimeType=message/rfc822;x-scheme-handler/mailto;
StartupNotify=true
Categories=Network;Email;
Name[cs]=Poštovní klient Thunderbird
Name[ca]=Client de correu Thunderbird
Name[fi]=Thunderbird-sähköposti
Name[fr]=Messagerie Thunderbird
Name[pl]=Klient poczty Thunderbird
Name[pt_BR]=Cliente de E-mail Thunderbird
Name[sv]=E-postklienten Thunderbird
Comment[ca]=Llegiu i escriviu correu
Comment[cs]=Čtení a psaní pošty
Comment[de]=Emails lesen und verfassen
Comment[fi]=Lue ja kirjoita sähköposteja
Comment[fr]=Lire et écrire des courriels
Comment[it]=Leggere e scrivere email
Comment[ja]=メールの読み書き
Comment[pl]=Czytanie i wysyłanie e-maili
Comment[pt_BR]=Ler e escrever suas mensagens
Comment[sv]=Läs och skriv e-post
X-Desktop-File-Install-Version=0.23


===============================================


cat /usr/share/applications/mozilla-thunderbird.desktop
[Desktop Entry]
Version=1.0
Name=Thunderbird
GenericName=Email
Comment=Send and Receive Email
Exec=thunderbird %u
TryExec=thunderbird
Icon=thunderbird
Terminal=false
Type=Application
MimeType=message/rfc822;x-scheme-handler/mailto;
StartupNotify=true
Categories=Network;Email;
Name[cs]=Poštovní klient Thunderbird
Name[ca]=Client de correu Thunderbird
Name[fi]=Thunderbird-sähköposti
Name[fr]=Messagerie Thunderbird
Name[pl]=Klient poczty Thunderbird
Name[pt_BR]=Cliente de E-mail Thunderbird
Name[sv]=E-postklienten Thunderbird
Comment[ca]=Llegiu i escriviu correu
Comment[cs]=Čtení a psaní pošty
Comment[de]=Emails lesen und verfassen
Comment[fi]=Lue ja kirjoita sähköposteja
Comment[fr]=Lire et écrire des courriels
Comment[it]=Leggere e scrivere email
Comment[ja]=メールの読み書き
Comment[pl]=Czytanie i wysyłanie e-maili
Comment[pt_BR]=Ler e escrever suas mensagens
Comment[sv]=Läs och skriv e-post
X-Desktop-File-Install-Version=0.23

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I am happy to say that the problem disappeared