Tool to backup Stumbleupon favourites

You can't.

In short, the answer would have to be... No.
We offer some tools along these lines, to those interested in promoting their sites, via our StumbleUpon Advertising programm
- CH, StumbleUpon employee

Not unless you sign up with their Paid Discovery plan which should give you analytics for all the pages under your site(s) that have been stumbled.

Looks like it's a way for them to offer an upsell on the StumbleUpon service.

Otherwise, comb through your logs for something with stumbleupon in the referrer URLs.

The first thing to do is go into Add-ons > Extensions > StumbleUpon: Preferences > Search & Tagging and make sure the "Save favorites & tags to a Firefox bookmar folder" checkbox is still checked.

If it is, try clicking the "Download Favorites" button to force a full re-download.

If that doesn't fix it, you may need to build a new Firefox profile to get things back on track. I know, on my system, upgrading from Firefox 3.6 to Firefox 4 broke my existing profile so badly that the address bar wasn't updating when I clicked links.

Thankfully, you can export your bookmarks and manually copy supplemental extension data like installed Greasemonkey scripts and Stylish userstyles if that proves necessary.