Try using Portable Firefox 28 to make that attempt to recover your data with the "found" Recovery Key. You can get it >>> here <<<. Scroll down to this date - 2014-03-18 Install it to you hard drive, then see if you can connect to that old Sync account to recover your data. If you can recover your data that way, use Help > About Firefox in that version to get it updated to Portable Firefox 33.1. Then you will need to update your Sync account. See this KB article - Sync your Firefox bookmarks, history, passwords and more

I am recommending doing it in that manner because Firefox the password manager was changed drastically a few versions ago, and I'm afraid that if you "go back" with a regular version of Firefox you may not get your passwords back in a usable form or "lose" them when updating to Firefox 33.1. And if you aren't successful you aren't risking messing up your current Firefox installation by messing around with that "go back" so many versions.

If you are successful recovering your Sync data, you can then connect your current installation to that Sync account to get your data into the new version 33.1 - and then just delete the Portable version.

Thank your for the quick answer. I tried what you suggested. Unfortunately it seems like I deleted my account completely because I can't reset my password. It tells me "Unable to locate your account. Please check your username." I guess I didn't read properly and acted prematurely. It probably won't work without rolling back the change on the server side. Furthermore I didn't set up a custom server so it would only work if the people at mozilla would do this. Plus, such an option probably doesn't even exit.

Nope, there is no "roll-back" on the Sync server.

From what I have read from other users posting here, you can't reset the password for the older Sync accounts any longer. After all, the old version of Sync has been just sitting there waiting for users to update their Firefox installation for 5 1/2 months now before is is shut down entirely.

I was still able to reset my account password for the old Sync last week. Accessing it via the web-interface and the new password worked as well. But since I couldn't sync because of a wrong sync key I must have decided to delete it because it wasn't useful anymore. Needless to say I regret that decision having found the correct key now. I will mark your answer as helpfull because of your quick and detailed information. Have a good day.

PS: Do I need to mark it as solved?