Add-ons written for opera and Google Chrome will now work on Firefox, because of Mozilla’s new feature. This means that the users will not have to stick to any specific browser for their add-ons.
However, all add-ons would require to get verified by Mozilla and then would be added through the official extensions portal of Firefox. Thus, a user cannot install the add-ons directly from the Chrome Web Store.
By using WebExtensions API, the developers would be able to maintain a tool across various browsers. It will basically enable interoperability between platforms.
Mozilla’s Kev Needham writes that many Firefox add-on developers usually maintain an Opera, Chrome or Safari extension with a similar functionality. The same code would run in multiple browsers as per the pre-defined behavior set by standards.
Before the add-ons get compatible with the new feature of Firefox, the existing add-on would require tweaking. The new feature is presently available in the developer edition. Gradually, this will get rolled out for the public.