The character &#9835; in the <title> section of an HTML page causes the title of the Firefox tab to become bold when it is the current tab, and appears to display in a different font. Selecting another tab can cause the title to spill over beyond the tab itself.

It appears that other Unicode characters cause this as well, and that different fonts are used depending on the Unicode character. Tested with the Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9a2pre) Gecko/20070117 Minefield/3.0a2pre ID:2007011704 [cairo] build of Minefield.