I see the same with WordPress 3.2.1 and FT Calendar 18.104.22.168, both with Firefox 4.0.1 and IE8.
<a class='large-prev' ref='" . $prev_month . "' href='" . $permalink . "?type=month&date=" . $prev_month . "'>" . apply_filters( 'ftcalendar-prev-arrow', '⇐' ) . "</a>
Changing 'large-prev' to 'large_prev' resolved the problem. I can't say I understand why. It appears that hyphens '-' are allowed in class names according to the CSS 2.1 spec, while underscores '_' are not:
"In CSS2, selectors (element names, classes and IDs) can contain only the characters [A-Za-z0-9] and [UNICODE] characters 161 and higher, plus the hyphen (-); they cannot start with a hyphen or a digit; they can also contain escaped characters and any Unicode character as a numeric code (see next item)."
Now, I was not able the find the class name defined in any file, so maybe the safest solution would just be to remove the reference to the class?