Comment 29 for bug 51554

More information:

I have:
Mozilla Firefox 38.0
Web page that displays this behaviour: https://github.com/cbm755/sympy/commit/cad689d3b82aa332a55b601d1a004f26fa6108f6 , search for "assert upretty( Symbol('Fhat') ) == u('F" (without the quotes) and the problem is directly after that.
CSS-requested font list: Consolas,"Liberation Mono",Menlo,Courier,monospace
Used font: Liberation Mono is used

Interestingly, the combining character is immediately followed by a span:
<span class="pl-s"><span class="pl-pds">'</span>F̂<span class="pl-pds">'</span></span>
Wild guess: the span right after Unicode COMBINING HAT disrupts Mozilla's rendering engine in some way. (It would explain why the issue is hard to reproduce.)
Detail: The pl-pds span changes font color, to a color of the same value; i.e. this may or may not contribute to the situation.