Updating mozilla firefox
While 32x32 pixel icons still work, obviously your add-on will look better if it provides a 64x64 pixel icon instead.
Fortunately, 64x64 icons are backward compatible and scale down well, so you can just switch instead of needing both sizes.
The first and most important thing to note is that starting in Firefox 10, add-ons are assumed to be compatible by default.
Unless you use the flag in your manifest, Firefox will no longer mark your add-on as incompatible after an upgrade to Firefox 10 or later.
This article offers details on changes in Firefox 4 that may impact existing extensions.
The status bar has been removed from Firefox 4, with a new add-on bar added in its place. If your add-on creates a new toolbar using an overlay, your toolbar may not show up.
It's possible that in the future the will be improved to fill the void.
The API for handling network redirects has been changed to be asynchronous; any add-ons registering in the "net-channel-event-sinks" category will need to be updated to use the new API.
A key difference with the new API is that requesting add-on metadata is now asynchronous instead of synchronous; this applies to add-ons using FUEL as well, so all add-ons that request metadata about add-ons will need to be updated.
If your add-on is discoverable only through the menu bar, you'll want to overlay the application menu as well.
There isn't a specific place to put extension menu items, so you should take a look at the menu and select the right place for your particular extension.
You can no longer pass Java Script objects between threads.
This renders the Thread Manager mostly useless for add-on developers, unfortunately, and there aren't at this time many alternatives.