Firefox (I tested 24 and 29) on GNU/Linux freezes page rendering when the size of this glyph U+EFF2 in the Junicode font is more than 1496%. It's reproducible on an Ubuntu 13.10 live CD. I couldn't get it to happen on Windows. I didn't try OS X. Occasionally Firefox crashes and bring up the error reporter, but I can't make it happen reliably; it might help to refresh and freeze the page a few times, hover over links, or highlight and drag after the display is frozen.

It happens with some other glyphs, like U+E044, U+E047, and U+E049 in Linux Libertine. It happens at slightly different font-sizes, but still around 1300%–1500%. Glyphs in between, like U+E046, seem fine.

You can still interact with the menu and other tabs, and clicking on links like the “refresh” link still works.