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.
A jar inside the XPI still works, but is no longer necessary, so that can make your development or build easier.
This was done mainly for performance reasons on slow OSes, and allows better cache invalidation, which also helps developers.
This article offers details on changes in Firefox 4 that may impact existing extensions.
They were most frequently being used to detect whether the document being displayed was HTML or XML.
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.
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.