Update: this change has been pushed back to Firefox 41 rather than 40.

Starting with Firefox 40 41, scheduled to be released in August this year, binary XPCOM support for extensions will be dropped.

Binary XPCOM is an old and fairly unstable technology that a small number of add-on developers have used to integrate binary libraries into their add-ons, sometimes to tap into Firefox internals (hence the unstable part). Better technologies have become available to replace binary XPCOM and we have encouraged developers to switch to them. From the original post:

Extension authors that need to use native binaries are encouraged to do

so using the addon SDK “system/child_process” pipe mechanism:

https://developer.mozilla.org/en-US/Add-ons/SDK/Low-Level_APIs/system_child_process If this is not sufficient, JS-ctypes may be an alternative mechanism to

use shared libraries, but this API is much more fragile and it’s easy to

write unsafe code.

Developers who rely on binary XPCOM should update their code as soon as possible to prevent compatibility issues. If you have any questions or comments about this move, please do so in the mozilla.dev.extensions newsgroup.