Almost immediately before the release of Firefox 1.0, the tabs were changed so that they were separated from the page by a thin line. (See bug 258884.) I’ve grown used to having them attached to the page as they were in 1.0PR. I guess the reason for the change is that there were complaints that the tabs didn’t work as well with web pages that had dark backgrounds. From what I’ve seen, the vast majority of pages use a light background color. I don’t blame the Firefox team for the last minute tweak—cleanup and polish before a release is great—but at the same time I miss the old style.
To restore the tabs to the way they were prior to the 1.0 release, I tracked down what exactly was changed in the bug fix. If you add the following to the userChrome.css in your profile directory, it will restore the tabs to the 1.0PR style. You may need to create the userChrome.css file.
border-bottom: 0 !important;