For quite sometime, we, Release management, have been using tracking flags to keep a list of important bugs for the release. Until now, for tracking, we used the "+" value to achieve this objective. This value carries different meanings:

• critical bug
• important regression
• important bug or issue that release management wants to be fixed

However, just using "+" does not allow us to differentiate between nice-to-have bug fixes vs must-fixes that block the release. To address this issue, we decided to introduce a new value to the tracking flags list: blocking. For bugs with tracking-firefoxN flag set to blocking, we will delay or skip a release.

Liz, Ritu & Sylvestre

http://release.mozilla.org/tracking/flags/blocking/2016/02/14/blocking-value-tracking-flags.html