The function of a drop down character box
was lost between Firefox 2.x and 3.x.
Considering that most of us use more than the low-ASCII symbol set from the first half of the 20th century, this was a great loss.
There are add-ins which solve the problem (but have conflicts with the dedicated Ubuntu hot keys, for instance) as well as the solution I use: I set my keyboard to IBM-German and have the full range.
If you need the symbols frequently, Windows XP and higher (higher? Worse-er?!) offer that language bar down by the system tray which lets you switch back and forth between layouts quickly and easily.
Of course, if it is just one or two symbols but they show up in your work constantly, you can use on-board applets in XP to add them to any TTF Font or higher.
If all else fails, pretend you're Swiss. They got rid of the ß.