A Firefox surprise from disabling dom.event.clipboardevents.enabled
Recently I wanted to copy some Grafana panels from one Grafana server to another, which in the modern web application way you do by viewing the JSON that defines the panel, copying it to the clipboard, going to the tab with your dashboard on the other server, and pasting in the JSON to overwrite the configuration of some victim panel. Old versions of Grafana had a handy 'copy to clipboard' button that did this for you, but in 7.5 you have to do it by hand. the menu item). Whenever I did this, I got mangled text, with most of the JSON elided and replaced with a '…' Unicode character.
(Via this article I found a suggestion of the Luminous addon, but brief experimentation suggests that something like Grafana is completely beyond the ability of an addon like this to do anything sensible with.)