Upgrading old overlay add-on to web extension AND maintaining both

Rob Wu rwu at mozilla.com
Tue Jun 6 19:56:54 UTC 2017

On Tue, May 9, 2017 at 5:24 PM, Boris Berkman <sbx.shopearn at gmail.com>

> Hi,
> First of all, I apologize if my email is sent to a wrong list or if the
> question(s) was/were already asked/answered, and I missed the answer when
> searching. In this case, please kindly point me in the right direction.
> Thanks!
> First - the current situation.
> So, we have an old, non-restartless overlay Firefox add-on, which we sign
> (at https://addons.mozilla.org/en-US/developers/addon/...), and then we
> distribute the signed version ( including updates -- to our members only!)
> from our membership-only web site. We have a lot of dedicated members (we
> are talking about hundreds-of-thousands active weekly members), and Firefox
> is our second (after Google Chrome) add-on platform, which we would like to
> support and maintain, as well as possible, without any service
> interruptions. We are getting close to making Web Extensions version ready
> to be released for Firefox.
> Now - actual questions.
> 1. We understand that - up to the current regular (non-beta, non-dev)
> version of the Firefox (53.0.2) - all those users of our Firefox addon who
> haven't modified their default options in the browser should still be able
> to use our overlay version in their browser [which is not set to use
> multi-process/electrolysis capabilities for any of them]. Is that the
> correct assumption or am I already missing something here (e.g., a need to
> add full multi-process support to the overlay add-on) ?

Yes, this is correct.

> 2. I understand (per https://developer.mozilla.org/en-US/Add-ons/Working_
> with_multiprocess_Firefox) that - "from Firefox 57
> <https://wiki.mozilla.org/RapidRelease/Calendar> onwards, WebExtensions
> will be the only supported extension type, and Firefox will not load other
> types". That is, no later than on 2017-11-14, we will have to make a
> transition - at least for all those members that run the latest version of
> the Firefox. Correct?

Yes. Possibly even two months earlier if your users use pre-release
versions of Firefox (e.g. Firefox Beta or Firefox Nightly). For the
schedule, see https://wiki.mozilla.org/RapidRelease/Calendar.

> 3. What is the smallest Firefox version number that had a reasonably
> complete support for WebExtensions? (We need to know that, so we could
> upgrade only the right people. Correct?)

WebExtensions was announced
to be stable in Firefox 48, but some APIs were already available in version
Make a list of APIs that you need, then look at the index of APIs and the
browser support at
Don't forget to test the add-on in the version that you're using to catch
bugs - see

> 4. In general, what is the Mozilla's "best practice" for supporting - in
> parallel - both WebExtensions for members who run the newer versions as
> well as overlay add-on type for older browser versions? If a user ever
> upgrades we should be able to seamlessly upgrade them to the WebExtensions,
> right? What if a user downgrades?

Firefox will install the latest compatible version of the add-on during
add-on updates. So the upgrade should be seamless.
If your add-on persists data or settings, you may want to migrate the data
to the WebExtension version of your add-on. To do so, use an Embedded
WebExtension (available since Firefox 51) -
The add-on is not automatically downgraded when the user downgrades their
browser version. It will still be installed (and be broken in unexpected
ways if the browser version does not provide the APIs that the add-on

> Thanks and sorry for so many questions,
> Boris.
> _______________________________________________
> Webextensions-support mailing list
> Webextensions-support at mozilla.org
> https://mail.mozilla.org/listinfo/webextensions-support
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