You would ony need to install the dictionary for all languages that you want to use. Then you can select tht correct dictionary via the right-click context menu in the text area if Firefox doesn't do this automatically. There is no need to install language pack because such a pack only contains the strings (labels) for user interface buttons and text and do not include a spelling dictionary.

You can see which dictionary is selected if you right-click in a text area and open the Languages sub menu. At least one dictionary should be installed and have a check mark to indicate that it is selected.

• open the "Add Dictionaries" link to install a dictionary if you do not have one.

The spelling checker is enabled if [X] "Check Spelling" in the right-click context menu has a checkmark.

You can enable or disable spell checking globally:

• Options/Preferences > Advanced : General: Browsing: "Check my spelling as I type"

You can look here for dictionaries:

• https://addons.mozilla.org/firefox/language-tools/

I have dictionaries for english and Swedish but what im talking about is that i want firefox to check my spelling in both languages at the same time. I don't want to have to right click and choose my language all the time.

Unfortunately that not possible. You can only have one dictionary active at the same time. There used to be the Dictionary Switcher extension to make this easier, but judging from the comment it looks that the extension isn't multi-process tabs/windows compatible.

• https://addons.mozilla.org/firefox/addon/dictionary-switcher/