There is this Swedish dictionary from the Open Office project.
http://extensions.services.openoffice.org/project/dict-sv
You will have to extract the two files (sv_SE.aff and sv_SE.dic) in the /dictionaries folder in that ZIP archive and install the dictionary manually by copying the files to the dictionary folder in the Firefox installation folder.
See: http://kb.mozillazine.org/Dictionaries#Installing_manually_to_the_application
The .oxt file is a ZIP archive and you can open that archive in a (ZIP) file manager and extract the sv_SE.aff and sv_SE.dic files in the dictionaries folder in that archive.
You may need to rename the file and give it a .zip file extension. Copy the sv_SE.aff and sv_SE.dic file to the dictionaries folder in the Firefox program folder (C:\Program Files\Mozilla Firefox)