switching the default monospace font on bmo yesterday highlighted a few issues with the fira-mono typeface that we’d like to see addressed before we use it. as a result comments are now displayed using their old font.
the following changes have been pushed to bugzilla.mozilla.org:
-  backout the fira mono font
-  add an “open sans” alternative for the mozilla skin (change your skin)
discuss these changes on mozilla.tools.bmo.