A user reported that in comparison to Mac OS the fonts look very ugly on Windows when integrated with ownCloud.
It seems not to be browser specific.
One possible solution would be to adjust this file /opt/collaboraoffice5.3/share/fonts/truetype/fc_local.conf according to the user.
For reference:
https://central.owncloud.org/t/collabora-code-ugly-fonts-with-windows-browsers/15258/2