Click on the three dots in the top left corner of the browser window and select "Settings"
In the left column, select "Languages", where you will find two windows "Preferred Languages" and "Spell Check"
In the "Preferred Languages" window, simply add the languages you want to check and in the "Spell Check" window, use the slider to turn the check on.
You will be taken to the page of the dictionary in question, where you can click on the "Add to Firefox" button. Once the file has been downloaded, you will see an Add-on Installation warning, in which you click on the Add button.
Once the dictionary is installed, you can switch to it as needed: right-click in a multi-line text field (in SingleCase, for example, a project note or report description), select Languages from the menu, and select the dictionary you wish to have active. You can select more than one dictionary. This will allow the spell checker to check spelling in multiple languages.