It is not happening that often that you have to accented characters or umlauts but it sometimes happens. Maybe you want to write a name like Søren or a unique German street name such as Bachstraße. If you never had to type those characters before you might wonder how it would be possible to write them down using your keyboard. They are obviously not printed on the keys of it.
One way to solve this problem is to use the Firefox extension Zombiekeys which adds exactly this functionality to Firefox. You have access to many, not sure if those are all, special characters that exist in European languages. Those include diacritics and ligatures. Unicode and Windows-1252 characters are supported as well. The key combinations have been directly taken from Microsoft Office which means that if you use them there, you will be able to use them in Firefox immediately.
- à , è, ì, ò, ù, À, È, Ì, Ò, Ù
CTRL+` (ACCENT GRAVE), the letter - é, Ã, ó, ú, ý, Ã?, É, Ã?, Ó, Ú, Ã?; ð, Ã?
CTRL+’ (APOSTROPHE), the letter; d or D - â, ê, î, ô, û, Â, Ê, ÃŽ, Ô, Û
CTRL+SHIFT+^ (CARET), the letter - ã, ñ, õ, Ã, Ñ, Õ
CTRL+SHIFT+~ (TILDE), the letter - ä, ë, ï, ö, ü, ÿ, Ä, Ë, �, Ö, Ü, Ÿ
CTRL+SHIFT+: (COLON), the letter - å, Å
CTRL+SHIFT+@, a or A - æ, Æ; œ, Œ; ß
CTRL+SHIFT+&, a or A; o or O; s - ç, Ç
CTRL+, (COMMA), c or C - ø, Ø, ¢
CTRL+/, o or O or c - ¿
CTRL+SHIFT+ALT+? - ¡
CTRL+SHIFT+ALT+! - €
CTRL+ALT+E - The Unicode character for the specified Unicode (hexadecimal) character code. For example, to insert the euro currency symbol (€), type 20AC, and then hold down the ALT key and press X.
The character code, ALT+X
The Windows-1252 character for the specified (decimal) character code.For example, to insert the euro currency symbol (€), hold down the ALT key and press 0128 on the numeric keypad.
ALT+the character code (on the numeric keypad)

Insert Unicode Characters Easily
Free font editor built into Windows
Leaked Hotmail Password Data Analysis
Firefox 3.1 Improvements
Save Websites With Mozilla Archive Format
Firefox Password Revealer
At last! Thanks a ton Martin
Or use Windows’ built-in Character Map: Start Menu > All Programs > Accessories > System Tools > Character Map (I have it at the ready in my Quick Launch bar)…
With it you simply find the character you want and copy it to the clipboard, or for many of the more common special characters it shows you the the keystroke combination to use instead, e.g. for ‘ß’ simply hold down ALT and enter ‘0223′ (on the number keypad, not the main keyboard). The more you use them, the more you remember the ALT+numbers combination… :)
Or you can simply use appropriate keyboard layout. For example, I frequently need to type Polish letters so I added Polish Programmer’s layout via Regional Options in Control Panel.
Now whenever I type in Polish, I simply hit my Language Bar and change the layout.
Once I do that I have all the polish characters such as ąęłóśćżź available via the Rigth Alt + Letter combination.
You should be able to do the same thing for German and any other language. Then it’s just a matter of learning to use the Alt + letter shortcuts fro appropriate characters.