When a WebExtension has a backslash character, "\", in its description field, Firefox considers the extension to be corrupt.
Created attachment 8702873 [details] backslash-in-description.xpi Attaching a WebExtension which manifest.json contains a description with the backslash character in it.
By contrast, an XPI extension may have a backslash in the appropriate field of its install.rdf, and the backslash appears in the add-on's description at about:addons. One example of such an extension is abcTajpu https://addons.mozilla.org/addon/abctajpu/
s/an XPI extension/a "classical" extension/
Backslash characters in JSON strings need to be escaped. The only characters allowed to follow a backslash in JSON strings are: "\/ubfnrt