If you have an extension installed permanently and then install one with the same ID temporarily, this is treated as an upgrade/downgrade (or it will be when bug 1359558 is fixed). This can come as a surprise, we can add a visual warning to developers to make sure they're aware of what's going on.
Not merely a surprise, but possible data corruption if the extension doesn't handle the DOWNGRADE reason, and leaves upgraded data in storage for the old version that can't handle it. "You are temporary upgrading to a newer version of this addon, which will get downgraded on shutdown. Make sure to handle that case or suffer data corruption consequences"
I realize I'm bikeshedding here but the warning from comment 1 sounds unnecessarily scary. Data corruption is only an issue if an extension does an irreversible data migration on upgrade, but that message makes it sound like a much broader problem.
Severity: normal → enhancement
Priority: -- → P2
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