Move your Xmarks to your own server
You'll need some server space and the ability to set up a WebDAV directory. Once you've set it up, you'll need to go to your Xmarks settings in Firefox and enter the url of your new directory. Then, still under Xmarks settings, hit the Synchronize Now button.
Bada bing, bada boom! You've set up your own hosted version of Xmarks! You'll have to enter the server information and hit sync again for each Firefox installation you use, but that shouldn't take very long.
If you'd prefer a visual walkthrough, check out Lifehacker's instructional video on the topic. If it still doesn't make sense, consider using your browser's built-in bookmark syncing tool.