It is a defect introduced with Firefox 1.5.1 and only impacts vBulletin sites (here's the official defect
report).
The problem is that vBulletin uses accesskeys for certain navigational functions (search accesskey in the source code for this page and you'll see it). Essentially accesskeys provide keyboard shortcuts for certain links on a page. So if you click Alt-1 on this page you'll load the LoT front page.
Notice the construction of how to use the accesskey. ALT and then a number. The Firefox defect is getting confused because when you try and type the € symbol hitting ALT- and then the first number of that code triggers the accesskey. The proposed fix is to make it so that number pad keys don't trigger accesskeys but that still won't help certain people.
This doesn't affect IE since IE doesn't support accesskeys.
Anyway, that is a long winded explanation just so you know why accesskeys are working if you want them in the future.
To solve this problem until a nicer solution is found you can use the advance browser settings to change the accesskey from ALT to META (you almost certainly don't have a META key, which means that accesskeys won't work for you but there won't be any conflict with the already assigned CTRL, SHIFT, etc. funcationality.
In your Firefox browser type the following into the address line:
about
:config
This will bring up a whole list of very specific advanced options settings. Don't change anything unless you know what you're doing. Near the top of the body of the page, above all the options you'll see a line that says Filter:. Type this into that box:
ui.key.generalAccessKey
This will filter the settings options down to just the one you want. The value will be set to 18. Double click on the option and a dialog box will open for inputting a new value. Change 18 to 224 and submit.
The option will now be bold and will stay that way since it is no longer set to the build default (this will make it easier to find in the future if you want to change it back).
The change is already in effect and there is no save or anything. Just close the browser, reopen, come to LoT and your ALT-whatever should be working.
Again, if you were using accesskey navigation (ALT-5 takes you to the FAQ, did you know?) that will no longer work.
Presumably all of this will be fixed in a future incremental release and you can keep an eye on the official bug if you want.