In their march to copy Google Chrome, Mozilla is moving Firefox to a multi-process architecture. The code name for this project is Electrolysis. As of this writing, this project's integration target for released levels of Firefox is at the end of 2015. Dates can always slip, and are likely to, but that target seems real soon now.
Frustratingly, Mozilla has been surprisingly quiet about this upcoming change, at least from a developer standpoint. For months the Mozilla Add-ons Blog has promised upcoming articles on the changes necessary for add-on authors, but as of this writing, nothing has appeared. What documentation does exist is, as usual, poorly written. The examples they provide aren't real-world enough for me to fully understand.
It frightens me that Mozilla should be so lackadaisical about evangelizing these changes. This architecture shift will affect the vast majority of add-ons in one form or another. I verified tonight in a nightly build that both Googlebar Lite and CoLT are affected by this change, the former being broken in a number of areas. It seems to me that Mozilla should shift their evangelism of this new architecture into high gear. Every developer who cares about application compatibility needs to be working on these changes sooner rather than later; otherwise, a ton of add-ons won't work properly come release day.