Recently, an AJAX based system of watching/unwatching pages has been implemented. It doesn't work for some pages on Farsi Wikipedia. An example is provided as a link.
This problem occurs in Firefox, and is because of the use of Zero Width Non Joiner character in the title of the page. CategoryTree extension had the same problem (see bug 9789) which I tried to fix, but left midway. As this new bug has a more widespread effect, I think either the AJAX code should be rewritten in a way that it can handle ZWNJ (u200C) character correctly on all browsers (e.g. by substituting it with another character) or this newly feature should be turned off until a newer version of Firefox is released with this bug resolved.