We released Firefox 21 just a couple of weeks ago. This means 22 is on Beta, 23 on Aurora and 24 on Nightly. Firefox 25 won’t be released until late October, so we still have a ways to go. However, this is a significant release because 24 is slated to be an Extended Support Release, so some major changes have been pushed after then to minimize any potential impact.

There are two major add-on compatibility changes to look forward to, and I’m giving you this heads up just so that you’re aware and plan ahead. Things might change as we move forward, and I’ll keep you posted about it, but you should definitely check out the Nightly builds in July, or the Aurora builds during the month of August. It’s likely that your add-on will be affected by these or other changes.

Australis

Update: as of September 19th, it looks like the Australis update won’t happen until at least Firefox 27.

This is a major theme revamp in Firefox that has been worked on for quite a while. One of its objectives is to simplify the UI, taking add-ons into account.

There are major changes coming to how toolbars work. There is some back and forth going on over the topic of removing the Add-on Bar completely. It doesn’t look like a final decision has been made, but in the best case it will be a harder to find customization target, and it the worst case it will be gone entirely.

The main toolbar will also have a dedicated area for add-on buttons and widgets, and it looks like user-created custom toolbars will also be gone. Overall, you should plan for a minimalistic toolbar UI. While most add-ons do this already, it’s possible that the API to add toolbar buttons will be very different, and there will be changes to be made for all of them.

You can do early testing of the Australis changes by installing the UX Nightly Branch. If you have any feedback from these tests, please share it in the comments below.

Session Restore

Like many other areas of Firefox code, Session Restore is getting the asynchronous treatment to improve its performance. If you look at bug 874381 and its dependencies, there are several changes to this module that will affect add-ons. Particularly, there are many add-ons that rely on private variables (starting with __SS) that will no longer exist.

There’s a list of affected add-ons in this blog post, and some of the developers have already been notified. However, you should check yourself if your add-on relies on any of these private variables and start migrating away from them, since they will be gone in 25.

Once we’re closer to the Firefox 25 release, we will have more information and hopefully better documentation to point you to. For now, I hope this heads up will be useful and you start looking into these big changes that are coming up.