I'm putting this in MediaWiki/JavaScript, but it's really a tracking bug across extensions, etc.
jQuery.browser was removed from jQuery 1.9. It currently works since we're using jQuery Migrate which puts it back. But that will be removed in time too (http://lists.wikimedia.org/pipermail/wikitech-l/2014-May/076340.html).
Ideally, you should replace it with feature detection. If that is not possible, you can use jQuery.client.
Version: unspecified
Severity: normal
Note that this was announced separately from jQuery 1.9 in general back in November 2012:
http://lists.wikimedia.org/pipermail/wikitech-l/2012-November/064280.html
http://www.mail-archive.com/wikitech-l%40lists.wikimedia.org/msg63751.html
It's not really an enhancement.
It's stuff that already doesn't work on test2wiki (jQuery Migrate isn't loaded there, so errors like this will pop up there first), and eventually will not work anywhere.
On the basis that there are no open sub-bugs of this, I'm marking it FIXED. Please re-open if there are still issues. Thank you everyone for your work!