Bookmarks disappear

I have a bunch of bookmarks in Firefox Portable. One of them keeps disappearing. I mean, I connect the USB, launch the software, open Firefox and the link is missing. I add it, it works. But just until I close the Firefox. If I open it, the link is missing again. It kept happening with one particular link (I think it was tinyurl.com website), now it's another perfectly normal, usual link. All other themes and pug-ins work fine. I have plenty of empty space on my USB. What is the problem?