Example demonstrating a potential Firefox bug where document.execCommand('paste')
doesn't work correctly when running as a web extension background page.
The extension using the ext1 folder does not work and logs an empty string.
The extension using the ext2 folder works and logs the text currently in the clipboard.
The only difference is that ext2 uses an incorrectly terminated <div> tag.
Both pages work when they are not being run as the background page,
i.e. open moz-extension://.../bg.html manually.