This is a bug in new Firefox versions such as version 15 and even version 16.So what is happening to you is that the inline spellchecker does not correctly traverse text nodes.
Try this simple solution first:
There is a Firefox add-on called "No-Squint".Disable it and the problem should go away.If not then try to completely remove that add-on.
If not then try this:
Try reseting the page zoom by going to
VIEW >
ZOOM >
RESET
Or you can just press
CTRL +
0
Where 0 is number ZERO.
If that also did not help then try this:
Turn off ANY ZOOM features in Firefox AND Windows.
If that did not work either then try this:
Try resseting all the Firefox settings back to the DEFAULT VALUES.
Cheers!