Why do some characters on my browser not show up?

[Jolinar]

When read some things, I get weird symbols sometimes.
� I am not sure if that will come up as I see or the real symbol. Anyways, that is what I sometimes see. Now I get this on quite a few pages, and I know other people can see what they actually are because one of the times I noticed this it was instructions on how to do something and a lot of people left comment saying it worked great. All I could see was symbols.
I'm using Firefox by the way - it updated today.
How can I fix this?

 

[Trotter]

Usually it is a character you don't have a font for, like a character in Chinese or something. You can download the packs for it... Google for them.