Hi, I will try to help you.
From a browser like Firefox go to this mozilla site
Choose one dictionary you want to add to Thunderbird, for example the Swedish one. But don't left-click the 'install' link directly, rather right-click it and choose 'Save Link As...' to download and save it on your hard disk, to a directory of your choice.
You need not be root to do that.
Then as user login open Thunderbird, click the Tools item on the Menu bar, choose Add-ons. You should find there an 'Install' button to be able to install the dictionary extension from the directory where you saved it from Firefox.
Restart Thunderbird, and your dictionary should be available for spell checking. This method is for a per user installation.