Likely timing of future Thunderbird Gecko builds

R Kent James kent at
Tue Apr 18 05:22:27 UTC 2017

TL;DR: Our branch of Gecko-esr59 may be the last version of 
mozilla-central that we support for Thunderbird.


In my post from September 17, 2015 "Future Planning: Thunderbird as a 
Web App" I proposed a likely future schedule. I said:

"Thunderbird 59:  Last and traditional XUL/C++ release. By this date, a 
reasonable possibility is that the Mozilla platform will no longer be 
usable for non-browser XUL-based development. This version of 
Thunderbird, in that case, would need to ship on a fork of Mozilla from 
the point where XUL-based development becomes untenable. This will also 
be our last major XUL-based release, and we will not attempt to keep 
current with the new non-XUL Mozilla platform."

Though 19 months have passed since that post, I still stand by that 
timing, and I think we need to seriously start planning our likely 
relationship to Gecko releases. Mozilla is talking about major changes 
beginning with Gecko 57, so we will probably need to branch Gecko at 
that point. We'll try to keep the branch as close to Gecko-esr59 as 
possible, so that while Gecko-esr59 is active we will be able to merge 
security changes. After EOL of Gecko-esr59 in late 2018, we will be on 
our own, responsible for continuing security updates to our mozilla branch.

That means that after Thunderbird 59, we will cease trying to maintain 
Thunderbird builds based on Gecko 60 and later, but will instead 
continue all further development of existing Thunderbird (which should 
be regressions and security patches only) using Gecko-esr59. After EOL 
of Gecko-esr59, that will require increasing effort to maintain security 
patches, such that this path is probably only viable for at most a year 
after EOL of Gecko-esr59.

That also means that during the mozilla-maintained lifetime of 
Gecko-esr59, we will need to devote all available effort and resources 
in a race against time to have a viable Thunderbird++ before the costs 
and risks of maintaining our fork of Gecko-esr59 becomes untenable.

What might change this? If the mozilla breakages at Gecko 57 are less 
than we fear, it might be possible to to continuing maintaining a 
Thunderbird build on Gecko for 60+. But we should make a hard decision 
at that point, and not just assume that we will continue the substantial 
effort to keep Thunderbird buildable on mozilla-central.

Per current schedule for 
merge of Firefox 60 to central is 2018-01-15, so we will need to decide 
before the end of the year whether to cease attempting to build 
Thunderbird on future Gecko versions.


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