[Merge] ~osomon/ubuntu-seeds/+git/ubuntu:impish-desktop-firefox-snap into ~ubuntu-core-dev/ubuntu-seeds/+git/ubuntu:impish

[Olivier Tilloy]

Thanks for the review Brian.
I agree that moving firefox to a supported seed to signify that the deb remains supported makes sense, however the wiki documentation¹ says about supported-desktop-extra "Packages that get a additional support for 5 years on a LTS", so it's not clear to me that it's the right place to put it.

¹ https://wiki.ubuntu.com/SeedManagement#supported-desktop-extra
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https://code.launchpad.net/~osomon/ubuntu-seeds/+git/ubuntu/+merge/408825
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