Harvest -> JIRA Sync by GG

One-click sync Harvest worklogs to JIRA with this browser extension!

You need to be logged in to your Harvest & JIRA account and configure the URLs on the settings page.
The extension popup will list you all worklogs you have in Harvest & JIRA for today. If the worklogs match in JIRA-<Ticket>, duration & exact comment they will be considered a match and listed in one line. You can navigate back and forth in time to also compare past days.

If an entry is only present in Harvest, and the comment begins with a valid JIRA ticket number, you will get offered a one-click sync to JIRA. There is also a convenience button for syncing all entries of a day at once.

Note: Repackaged from Chrome add-on. Adding here for easy installation in Firefox.