Rated 5 out of 5 stars

Extract the xpi, find pageInfoOverlay.js, then replace function GetCache with below code. This works with static files, but the html size still being shown as unknown.

// At the top of the file
var gCacheService = Components.classes["@mozilla.org/network/cache-service;1"].getService(Components.interfaces.nsICacheService);

// Replace function GetCache
function GetCache(url, callback)
{
const ACCESS_READ = Components.interfaces.nsICache.ACCESS_READ;
try {
var httpCacheSession = gCacheService.createSession("HTTP", Components.interfaces.nsICache.STORE_ANYWHERE, true);
httpCacheSession.asyncOpenCacheEntry(url, ACCESS_READ,
{onCacheEntryAvailable: function(entry, access, status) {
if (entry)
callback(entry);
else {
var ftpCacheSession = gCacheService.createSession("FTP", Components.interfaces.nsICache.STORE_ANYWHERE, true);
ftpCacheSession.asyncOpenCacheEntry(url, ACCESS_READ,
{onCacheEntryAvailable: function(entry, access, status) {
callback(entry);
}}, true);
}
}}, true);
} catch(ex) {
console.log(ex);
callback(null);
}
}