OK, I know exactly what's wrong and I thought it kind of annoying that you are just stating it as a "problem" with Firefox.
Before I explain, though, all standard letters on your keyboard are ASCII, the characters you are trying to type are Extended ASCII characters, so ASCII is not blocked at all.
Right, you've stated it works in IE? That's because there is a bug in IE, not in Firefox. Firefox is behaving as any browser should.
In HTML it is possible to assign access keys to tags included on the page, this is to help navigating a page. Access keys are very similar to the short cut keys that you use daily in any modern application; E.G. Ctrl+C (Copy).
The access key behaves like this: <a href='someurl' accesskey='KEY'>some text</a>. Now, if you were to put the letter x there in place of KEY, the browser would now automatically follow that link when you press Alt+x on the page containing the a tag.
Where does this come into the forum? Well, with Jelsoft being a fairly big company and a leader in forum software, they clearly wanted to allow users to use shortcut keys with their product, just like many other companies, so they decided that they would add access keys to some of the more commonly used functions on their forum software. These functions are listed below:
Alt+1 - Go to the Index.
Alt+2 - New posts.
Alt+3 - Show printable version.
Alt+4 - Search.
Alt+5 - FAQ.
Alt+u - Jump to username box.
Alt+p - Jump to password box.
Alt+c - Check "Remember Me" box.
Alt+s - Log in.
So, therefore, as Firefox is doing its job as a browser correctly, when you press Alt+233 it recognizes it as Alt+3 (3 being the final key you pressed) and takes you to the printable page.
It may be annoying and causing you problems with typing Extended ASCII characters, but it certainly is not a bug.
Hopefully that helps :)