Linux comes with a bitmap font called Helvetica. It looks terrible; nothing like the Helvetica that comes with Mac OS, except at large sizes. I'd change the if...else to a switch and have a few simple options: 'sans-serif', 'serif', and 'default', which would use whatever the wiki has set default.

Also, why all the empty catch statements? I tried removing them all and only got a couple "foo is null" warnings which I fixed by wrapping the appropriate code in if (x !== null) {...}. (I use === and !== because they're more efficient than == and != and don't do that weird type coercion.)

Interesting---You're trying to learn JavaScript by writing in GreaseMonkey, and I'm trying to learn GreaseMonkey by reading some scripts (haven't written anything yet) :-)

For learning JS, check out Douglas Crockford's articles[0] which explain how to avoid some of JavaScript's pitfalls. Also nice is JSLint[1] which warns about things that may have unpredictable results. For example, JS doesn't have block scope, only function scope, so any var declaration is 'hoisted' to the top of the function, even if you typed it in a loop or conditional. So having var i = 0; in several for loops within the same function (or outside any function) is an error: put var i; before the loop and i = 0; in the loop.

[0]: http://www.crockford.com/javascript/
[1]: http://www.jslint.com/