Leading change in visual editor

For no reason I can discern, the leading and default font has changed in my visual editor, and it’s much more difficult for my editors to read. I’ve combed through every settings document I can find in WP and Tiny MCE, and I can’t find out where this would be controlled, or how it could have changed. Tiny MCE forums sent me here.
 
A) TinyMCE and WP is up to date
B) I’ve exported TinyMCE settings from my old dev site and imported them to my live site, but the problem persists
C) Problem is across all users, not just one account
D) Problem shows anywhere TinyMCE does – not just post editing, but user bio editing, etc.

Any help is welcome. It’s been causing me problems for days.