I am not aware of an official decision on this just yet, but we should make sure that we can build against the 10.11 SDK while targeting 10.12 and above.

Try seems to have no issue with this change:

https://treeherder.mozilla.org/#/jobs?repo=try&selectedTaskRun=F5v1oM1ETAWKcNdA04nWnw-0&revision=ba4daceb334fcacbbbad713452452f459ce24fef

Backed out per Slack discussion so an Intent to Deprecate email can be sent to dev-platform first.
https://hg.mozilla.org/integration/autoland/rev/9dd2ca272d112459744246c4b4ca6ae9792d74ee

Intent to Deprecate email was sent out:
https://groups.google.com/d/msg/mozilla.dev.platform/anKO7Z_Mb_g/6AYIQSsXCAAJ

Balrog was also updated to freeze Nightly updates for macOS 10.9-10.11 users and verified working by QA. I think we're good to proceed with this now!

https://hg.mozilla.org/mozilla-central/rev/3a7d477d48d0

Verified by using the latest Firefox 79.0a1 (20200617093637) from archive on macOS 10.9.5, 10.10.5 and macOS 10.11.6. An error message is presented when opening Firefox Nightly .dmg and .pkg files. Attached a screenshot with the icons and the error message from macOS 10.11.6.

Opened the same build on macOS 10.12 and macOS 10.15 and they seem to work properly.

Also when opening Firefox from the terminal Crash Reporter is opened having this error message presented in the terminal. This is the crash report for macOS 10.10 and macOS 10.11. Is it expected to have the crash reporter and not the same window with the error presented when opening Firefox normally?

(In reply to Alexandru Trif, QA [:atrif] from comment #9)

Also when opening Firefox from the terminal Crash Reporter is opened having this error message presented in the terminal. This is the crash report for macOS 10.10 and macOS 10.11. Is it expected to have the crash reporter and not the same window with the error presented when opening Firefox normally?

Yes, it is expected to not see the same dialog. I wasn't necessarily expecting for the crash reporter to start, but the app gets aborted, so it makes sense. Since these crash reports are easy to filter out, I don't believe there is anything we need to do here at the moment. If this becomes a problem for some reason, we could add the same version restriction to the crash reporter, which would prevent it from launching too.

(In reply to Stephen A Pohl [:spohl] from comment #10)

(In reply to Alexandru Trif, QA [:atrif] from comment #9)

Also when opening Firefox from the terminal Crash Reporter is opened having this error message presented in the terminal. This is the crash report for macOS 10.10 and macOS 10.11. Is it expected to have the crash reporter and not the same window with the error presented when opening Firefox normally?

Yes, it is expected to not see the same dialog. I wasn't necessarily expecting for the crash reporter to start, but the app gets aborted, so it makes sense. Since these crash reports are easy to filter out, I don't believe there is anything we need to do here at the moment. If this becomes a problem for some reason, we could add the same version restriction to the crash reporter, which would prevent it from launching too.

Thank you! The issue is verified based on comment 9.

We have relnotes for both Release 78 and Beta 79 referencing this drop in support and migration to ESR78.

It makes sense to drop support for macOS 10.9 and 10.10, as there is very few reasons to stay on 10.9 (old macOS design) and 10.10 (SIP not present). but there is no point to drop support for macOS 10.11 considering it is the last version of macOS without SSE4.1 requierement, there are much computers stuck in this version officially and unofficially.