Changes have been made in Git (FRAMEWORK_5_2):

commit 3d14ccc7193f20855229b45af8819c266cc4bf77
Author: Jan Schneider <jan@horde.org>
Date:   Thu Dec 17 17:07:31 2015 +0100

     [jan] Fix setting title with newmail count in IE11 and Edge (Bug #14189).

  imp/docs/CHANGES |    1 +
  imp/package.xml  |    2 ++
  2 files changed, 3 insertions(+), 0 deletions(-)

http://github.com/horde/horde/commit/3d14ccc7193f20855229b45af8819c266cc4bf77

Changes have been made in Git (FRAMEWORK_5_2):

commit 7c862625fba9b9c14db4cde865ab9bd2b6f66f4d
Author: Jan Schneider <jan@horde.org>
Date:   Thu Dec 17 17:03:14 2015 +0100

     Set title instead of favicon for IE11 and Edge too.

     Both actually support canvas and the favicon link is set 
correctly. But for some unknown reason, they don't support setting 
favicons dynamically to data. Oh my.

     Bug: 14189

  imp/js/external/tinycon.js |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://github.com/horde/horde/commit/7c862625fba9b9c14db4cde865ab9bd2b6f66f4d

Changes have been made in Git (master):

commit 63a021a45eccab5c92b3e9ce7749d77049198a8a
Author: Jan Schneider <jan@horde.org>
Date:   Thu Dec 17 17:03:14 2015 +0100

     Set title instead of favicon for IE11 and Edge too.

     Both actually support canvas and the favicon link is set 
correctly. But for some unknown reason, they don't support setting 
favicons dynamically to data. Oh my.

     Bug: 14189

  imp/js/external/tinycon.js |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://github.com/horde/horde/commit/63a021a45eccab5c92b3e9ce7749d77049198a8a

Thank you for your hint, I believe I have found it:

it is tinycon.js, under js/external - if I understand correctly, it 
does not use the browser detection function in Horde_Browser (which 
has code for detecting IE11), but its own.

There is already a patch suggested  here:

https://github.com/tommoor/tinycon/pull/81

which I have expanded to also catch MS Edge on Windows 10 (see patch 
attached). Please note that I'm no expert in the matter, this seems to 
work here but I have no thorough understanding whether it could break 
other things (i.e. detection of other browsers).

This would be done in Horde_Browser, probably in the ::match() method.

This:

[mms] Display unread message count in title via favicon bubbles, if browser
       supports.

does not work under IE 11 (11.0.9600.18059) - just the standard 
envelope favicon is displayed, no bubbles. It works fine under other 
browsers, including Firefox and Chrome. Apparently, IE11 is considered 
  a "supporting browser", because the previous system (display number 
of unread mails in the page title) is not used either (but can be 
activated when I switch the "document mode" to "IE10" (or lower) in 
the IE developer options). As a workaround, I would be thankful for 
any hint where in the code I can delete IE11 from the list of 
supported browsers...

Best regards,

Martin