Now I'm getting similar behavior just opening a bunch of Bugzilla tabs from Bugzilla. https://share.firefox.dev/3iDK6en
Bug 1722722 Comment 12 Edit History
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Now I'm getting similar behavior just opening a bunch of Bugzilla tabs from Gmail. https://share.firefox.dev/3iDK6en (edit: "from Gmail" not "from Bugzilla")