XKit 7.4.1 is now available
This version adds support for the new SSL setting that Tumblr added and broke XKit earlier. It is available for Firefox, Chrome and Safari. I’m working on the Opera port.
On Chrome, you can wait for Chrome to update XKit or follow the instructions here.
On Firefox and Safari, just go and re-download XKit, then restart your browser. Your settings will be preserved.
You should also perform a XKit Control Panel > Other > Update All for XKit Main extension to update unless you don’t want to wait for XKit to update that extension.
If you don’t plan on using SSL setting, you don’t need to update, but it is recommended.