Mozilla is working on Firefox's multi-process architecture that it calls e10s and interested users can enable it in some versions of Firefox already.
If you have tried it already you may have noticed that some sites or add-ons that you are using are not working properly when e10s is enabled in the browser.
If you are interested in the progress that is being made in regards to add-ons, then you may want to head over to the
Are we e10s yet website which lists popular add-ons for Firefox and their compatibility state.
There you find listed HTTPS Everywhere for example which is listed as not compatible right now. Each add-on that is not working at all or only partially has a bug assigned to it that is used to keep track of its compatibility progress.
