Firefox version of extension was disabled after the latest release of Firefox
After the latest version of Firefox is released, for some users the extension is automatically disabled / removed.
Solution: If the extension is disabled you can enable it again from the "Add-ons" menu in Firefox. If it is removed, then you will have to install it again from the following URL: https://addons.mozilla.org/en-US/firefox/addon/jira-assistant/
But unfortunately, in store as well it is disabled. So till new version is uploaded again to Firefox addons store, their is no way around this to use in Firefox. I assume a new version will be available for Firefox in a couple of weeks from the date of this issue being raised.