[Osiris]

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, í, ó, ú, ý, Á, É, Í, Ó, Ú, Ý; ð, Ð
  CTRL+’ (APOSTROPHE), the letter; d or D
• â, ê, î, ô, û, Â, Ê, Î, 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, Ï, 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)

[TheMajor]

All those characters can be typed without any extensions. When using the US-International keyboard setting in Windows, some characters can be typed using dead keys, and others using AltGr (right Alt key, or the left CTRL+ALT).

Some examples

Using dead keys:

<press ", then press e afterwards> outputs ë
<' & u> outputs ú
<^ & i> outputs î

Using AltGr (right Alt key, or CTRL+ALT):

<AltGr + s> outputs ß
<AltGr + 5> outputs €
<AltGr + n> outputs ñ
<AltGr + z> outputs æ

You can always use SHIFT:

<Press ", then press SHIFT + A> output: Ä
<SHIFT + AltGr + z> output: Æ

Much easier and faster than typing Alt+decimal character code.